The restrictions you mention are for boarding your plane in the UK If the Polonium was taken on board in Russia, there would be no such restrictions.
When I went on holiday in September there wre all kinds of stupid restiction on departure from UK, but there was nothing on my return.
So any mad terrorists can enjoy a nice holiday before they go to meet their maker.
Then again, if we weren't constantly on alert and told there are nasty people trying to kill us, then we might wonder why we are fighting two wars in sand covered lands far away.