(bringing up a popular example often mentioned here on FT)
It is also very possible for terrorists to bring explosives onto an aircraft by hiding them inside their body. Would you like to submit to a rectal exam every time you fly?

A resonable compromise must be found between full-blown 100% security without freedom and a comfortable, practical way to travel long distances. Banning every possible item or substance that could possibly be used to harm anyone is not resonable in my mind. Fighting terrorism should be done through intelligence and policework, which seems to be what they used in the UK, not by assuming that all travellers are potential terrorists and treating them like criminals.