HMRC have published a note about offshore bank accounts. It starts by saying:
There are many wholly legitimate reasons for opening an offshore bank account. UK residents may have overseas business or personal interest or family ties or they may simply be attracted by the products offered by an overseas financial institution. It would be wrong to assume that all such relationships are tax motivated or that there is any tax loss arising.
This admirably balanced statement may be contrasted with the rest of the note which sets out penalties for a harsh notification regime for foreign bank accounts which are opened in certain (as yet unspecified) jurisdictions.
Failure to notify HMRC of the opening of such a foreign bank account will involve serious penalties – and if you fail to disclose the income from the account as well (which I guess is pretty likely if you have chosen not to disclose its existence to HMRC) the penalties can rack up to 200%.