You can buy a small, petrol-powered robochopper which might cost under $60k, carries up to 50lb of payload and could fit into the back of a van or small lorry all ready to fly. Pull up somewhere quiet the other side of St James' Park, launch the bird and drive away.
It has controllers and software capable of automatically flying a 3D waypoint route, apparently good enough to stay inside an area no more than six metres wide and one metre up and down.
So the robochopper zooms along gaily without further input from you just above the rooftops and trees, crosses Horse Guards and comes in where you choose - roof, window, maybe doorway of Number 10, accurate to maybe +/- 3 metres. Fifty pounds of explosive device goes off. Job done.
Maybe not, though. You've definitely got your headlines but you're fairly unlikely to have bagged your prime minister, unless you could predict in advance - almost to the second - just when he'd be between the door and the armoured car (even his press secretary probably can't predict it that closely). Even then, a bit of wind, an alert close-protection man, and you're out of luck. He'll be into the car or the building before the explosion.
One can, of course, destroy or partly destroy buildings and armoured limos with less than 50lb of explosives - the Brighton hotel bomb was smaller than that - but the stuff has to be carefully placed and prepared, probably inside the target. If all you can manage is crashlanding the bomb within an area six metres across you need to be thinking in terms of hundreds of pounds warhead weight for these targets.
The new, US "miniaturised" GBU-39 smartbomb - ultra-accurate, designed to get a kill using the minimum necessary amount of bang - is in the 250lb class. Most of this is actually the steel penetration case, which would let it punch inside No 10 or the PM's limo before detonating its 50lb charge. But the robochopper can't hit hard or fast enough to get this kind of result, even if it could lift the weight.
Probably the best plan would be to make a 50lb large-diameter shaped charge of the sort now being used in roadside "superbombs" in Iraq (it isn't too hard) and hang that under your robochopper. Fly the chopper at the PM's vehicle. To have any serious chance of success you will need to manually pilot the weapon in; GPS will quite likely miss altogether even if the vehicle is obliging enough to remain stationary, and with an explosively formed slug you need to be fairly accurate.
A $50k-60k robocopter can be handled remotely using a live vid feed, but now you've got serious problems. Your drone is no longer electronically stealthy - it can be detected a damn sight more easily than a blackbird. It can also be meddled with, and it will be. Even ordinary military patrols these days deploy sensors and jammers which can warn of an elctronic threat in advance and perhaps cancel it out.
COMMENTS
re "I think" by AC
Quite right, bro.
And you don't even need as much as Plan B.
A while back there was a hyped up big scare here in New Zealand when postal workers found a mysterious white powder in a couple of envelopes - the worry was that it might have been Anthrax and it caused Nationwide alerts, decontamination procedures, hospitalisation of those who'd come in contact with the white powder and a media frenzy for a while.
How much would that have cost? Probably around a (NZ) dollar per letter including postage, the envelope and some random white powder. No one was actually harmed but the panic it caused, hyped up by the Allies of Terror (media and politicians), was enough to satisfy anyone intent on spreading fear. That was over just a couple of them.
It's got to the point that no one even has to die. Some numpty makes a "car bomb" with no chance whatsoever of actually detonating and the papers and news reports are full of "if it had gone off it could have [insert hyberbolic description of carnage here]" and screaming about near misses and calamity-barely-averted etc etc.
Whatever happened to the days when the terrorists actually had to KILL people to spread panic? Since when does "what if it had been a real car bomb/thermonuclear device/vial of cultured Bubonic" justify a nationwide furore?
And now its "Jeez, what if They try to emulate the superpowers and build fiendishly complicated devices, better sound the klaxons and send out the press releases!"
If I were the terrorists, I'd charge the media royalties every time they linked the name of my organisation to the actions of some nutter or to wild speculations about what we *might* do if we ever got out hands on a portable disintegrator ray and then use the enormous revenue that would create to retire to the Bahamas - after all, it's not like the terrorists actually have to lift a finger these days, they might as well kick off their shoes, stick their feet up and order a banana Daiquiri.
In New Zealand in1982, a rather disaffected young punk rocker named Neil Roberts blew himself up in an attempt to destroy the Wanganui Computer Centre (the repository of our police records at the time). All he managed to achieve was to make a bit of a mess for people to clean up.
Back then, most of the country just laughed and said "what a fucking idiot".
These days there'd be six months of "Oh noes! He might have destroyed us all and plunged us into the Dark Ages" and speculations that he might have connections with Al Qaeda because one of the guys in his class was a Muslim (or knew a Muslim).
Jeez, what if he'd built a rocket launcher out of some lengths of pipe and left-over Guy Fawkes' Night rockets! Calamity!
I think
for all those in the fancy gadget camp remember this
"Keep it simple, stupid"
Plan a: use a dozen remote controlled aircraft to rain explosives down on a crowded area (a pro something or other rally).
Plan b: place timed explosives in or around village pubs also combine this with a suicide device in the queue for check in on a busy flight to the US at Heathrow.
See this was an interesting quote
"we have to fear is fear itself."
But I think we need the whole quote
"This is pre-eminently the time to speak the truth, the whole truth, frankly and boldly. Nor need we shrink from honestly facing conditions in our country today. This great Nation will endure as it has endured, will revive and will prosper. So, first of all, let me assert my firm belief that the only thing we have to fear is fear itself—nameless, unreasoning, unjustified terror which paralyses needed efforts to convert retreat into advance. In every dark hour of our national life a leadership of frankness and vigour has met with that understanding and support of the people themselves which is essential to victory. I am convinced that you will again give that support to leadership in these critical days."
Our government now spread nameless, unreasoning, unjustified, terror in aim to paralyse us so that we retreat into our own houses, cowering in the dark, stay blind to the twisting changes that they enforce. Turning victory in WW2 and the cold war into defeat.
yah, it'll never happen, my arse
that somebody could build a deadly weapon from hobby grade gear is about as probable as a couple of hijackers with box knives flying a 747 into a building...oops!
Lack of imagination is what Saudi's pay the media for these days?
Let's look into a nice friendly gentleman who starred in the American version of your "Scrapheap Challenge" (Junkyard Wars over here). who builds his own pulse ramjet motors:
http://www.interestingprojects.com/cruisemissile/
I've got his downloadable book but never received the full pay one before he got shut down, notably by a good "tolerant" government who uses tax law to lean on the "white" guy but lets the vile hate spewing terrorist rabble rousers go free. This gentleman was a decent man, inventor, and good all around backyard builder. He was helpful and answered questions from folks about his designs and research all the time until he tried to prove that yes, there is a legitimate concern that needs to be addressed. But that's not what insiders really want...they want people to relax, to believe there are no terrorists or racist, muslim extremists and hounded this guy out of business.
So you can take that holier than thou, can't be done, smarminess attitude and cram it. It's about time to start being part of the solution, not the problem. The solution is, wipe out evil where it exists, instead of coddling it, or kissing it's arse. Failing that, ridiculing the true evil instead of those defending you from it would be a great start.
So if there's nothing to fear but "fear itself", let's see Monsieur Page and the rest proudly wear some Danish "Mo-bomb-hed" T shirts and a nice friendly "Islam is da Shiite" bumper sticker on their car/scooter and prove how tolerant and peace loving the "religion of peace" truly is.

Requirements Checklist for Choosing a Cloud Backup and Recovery Service Provider
Cloud based data management
Agentless Backup is Not a Myth
SaaS data loss: The problem you didn’t know you had
What you need to know about cloud backup