The guys that make robots for Robot wars are mostly pretty good engineers and their biggest problem is reliability, it would be the same for a terrorist with his 250 quid robot. Making it truly autonomous though wouldn't be as much of a problem as the thing getting far enough away from the terrorist before starting it's kill spree. To be worth while doing it has to be in a city or town centre, I can't imagine any thing like that getting far in central London or almost any other town centre for that matter. In London it will get done for not having a current tax disc and not having paid its congestion charge.