Agree with pred; a nation that can't defend itself against any aggressor is useless.
You also have to consider the wider implications of scrapping Trident and how it would be viewed by our allies, particularly the US. For too long Europe has taken US defence for granted. There are already pressures on the US to increase defence commitments elsewhere at the expense of Europe, particularly in the Pacific rim.
If we get rid of Trident, it reinforces the message that Europe is going soft and the Americans will eventually assume that Europe isn't worth defending.
If we aren't prepared to spend resources to defend ourselves, is it morally right we should depend on them?