The biggest single danger is not a car, but a plane.  Bought legitimately so as not to cause any suspicions, and parked up at Headcorn Airfield, south of Maidstone, a Twin Comanche could be packed with high explosives and make it to the Houses of Parliament in less than twenty minutes.  Coming in fast at 170mph at zero altitude - 100', before any defensive system could be scrambled, it would be all over.  Hit at the right angle - 30 degrees to the main windows, it would bring the roof down, no problem, and with so many political heads wiped out in one go, the entire country could fold where military personnel would be drafted in to take control until such times that another Government would be able to be sworn in.  Such an attack would have to be well planned - it would have to be something major like a vote being taken on Scotland's possible indipendence, for instance.  This country does have an air defence system, but whether it could act quickly enough to stop a kami-kaze style attack is questionable.  I'll not write too much more - you don't know who's reading this!:-D  Overactive imagination?  I surely hope so! :^O