We need a nuclear deterrent, and having a sea deterrent is probably the best option available. It's more expensive than a silo, but if we had one of those on British soil too many people would complain.
That said though, it wouldn't hurt to downgrade to maybe 3 submarines instead. Nonetheless, we definitely need a minimum nuclear deterrent. It's the only thing which keeps us in the security council and allows us to be considered a great power.
I'd rather have a weapon and never have to use it than meet the day that I have need of it and don't have it.