Almost all US persons moving to the UK will not be domiciled within the UK for UK tax purposes. As a consequence, UK tax (at rates of up to 40%) is payable on income and gains arising in the UK (and Republic of Ireland), or remitted to the UK.
The Channel Islands & Isle of Man are offshore for UK tax purposes, but part of the British bank clearing system. Therefore, an account held there is for every purpose identical to an onshore UK account EXCEPT that the interest credited will remain free of UK tax (unless the interest is brought to the UK). This is a standard tax planning technique for everyone moving to the UK. It is not tax evasion, illegal or unusual. HSBC, Barclays, NatWest and HBOS (indeed all major UK banks) all have branches in the Channel Islands & Isle of Man precisely to service this kind of customer.
Now of course there is nothing to stop everyone volunteering to paying more tax than they need, but with my helpful cap on this is always worth saying.
Those coming for shorter periods, such as less than 3 years, can get even larger tax savings by banking offshore.