I left half a bottle of water in my bag and it was spotted in the Xray and taken out. I said I would drink it there and then rather than throw it away. The security guard said that I couldn't do that and refused to let me have it back even just to drink it.
I questioned this and got the 'rules is rules' answer.
I think its just an excuse to sell you a 0.5 litre bottle of water for £1.20 (more expensive than AVGAS!). Notice there are no drinking fountains anywhere in British airports, unlike US ones.
BAA monopoly!?