Hmmm, not sure about the £5 car parking fee as a deterrent, as mentioned. In the good olde days of the IRA they'd never have raised that sort of capital though old-fashioned pub collections. But now - why, these terrorist buggers are RICH - with money we've given them through buying oil off them or contracting them to build roads in the fight against Russian invaders.
My favourite game is to smuggle tubes of chain grease back from Germany. You can't get it readily in UK and it lasts longer on bike chains than the 3-in-1 stuff. I've never had it confiscated yet, though you have to separate it from more obvious "terrorist equipment" - like puncture repair glue, which has been confiscated. Or the plastic zip-ties, which, apparently, can be used to tie someones hands behind their backs ... as can luggage straps, or, if you're feeling particularly adventurous, frilly knickers ...