Hiya! Well, I technically have an offshore bank account with HBOS, but I live in the Isle of Man and the rules are a bit different for us.
Basically, whilst your funds are held in an offshore you're not liable for UK taxes on the capital and the interest. Your money can just sit there and grow. As you've probably sussed out, the headaches come when you move the funds back to the UK, to buy a house or whatever. I believe, in that case, it's only the interest you've accrued that will be taxable, but I can't verify that with personal experience. I think that you'll still declare the interest as income on your self assessment, but the taxes will be deferred until you cash out. Please double check that information, though, since my point of view is of someone already living in a tax haven who doesn't have to worry too much about the UK bit.