My thinking is: the benefits of Britain taking a step forward toward deprolifiration outweigh the possible risks. If Britain took the important step, it would set an impressive precedent, and if done right, give a boost to the nuclear disarmament movement. So you ask, what about the necessity of a nuclear detterant. Well, for one thing, the UK is not under threat, even remotely, from anyone that I know of, and is not likely to be so for a long time. Everyone in Europe is more or less buddy buddy, and the chances of another WW2 scenario erupting are very remote. Even then, there are two other countries, France and America, who's interests are close enough the UK's that they can fulfill the requirements of "don't you dare look at me like that, I've got The Bomb". Sure, not too many people prefer to leave their defense, even potentially, in the hands of others, but in any real world scenario I think Britain could be considered as good as armed (with nukes).
In any case, if it were to happen, Britain would be perhaps the only country on Earth that could talk about nuclear disarmament from moral high ground.