The latest advise I got from the UK tax office is that wheter you bank on or offshore makes absolutely no difference to whether you are considered resident in the UK. However if you bank onshore you will be taxed on any interest arising.
The 91 day rule is the important one, but you also have to prove that you are non resident, one of the things they look at being your life style. If your partner and Children live in the UK and you spend 90 days a year in your home with them the revenue will probably argue that your life is really in the uk and class you as resident there.
If on the other hand you are single, have no property in the uk where you can live (either you don't own any or it is let long term and you don't have access to it, although you would be liable for tax on the rental income) then they will be more inclined to believe you when you say you have left the country.
Start off with a google search for "UK residency for tax purposes" or something similar and sit back for hours of enthralling reading. All of the rules and required forms are down loadable from the HMRC web site.