There's a big difference between a nuclear sub, one which relies on nuclear power for propulsion, and a nuclear missile sub, which is armed with nuclear weapons. I agree that, other than an all-out nuclear holocaust, the missile subs are useless except as a deterrent. However, they can be fitted with non-nuclear, Tomahawk-type cruise missiles for more conventional warfare. In fact, such missiles were launched from submarines during the latest Iraq war, with good effect.
But nuclear powered submarines, and other naval vessels, are probably here to stay. While more expensive to build, they are cheaper to operate, can stay at sea for long periods, and, in the case of submarines, are far more stealthy than diesel/electric subs. And despite the protests of the anti-nuclear crowd, they are safer for both the environment and their crews.
Whether or not the UK, or the US for that matter, actually NEED as many as they already have, though, is a different question altogether.