There is a little something called the 'global ATM alliance'
http://en.wikipedia.org/wiki/Global_ATM_Alliance, and it allows people in the alliance to withdraw money fee-free worldwide.
With this in mind, one of the best US accounts to use in the UK is from Bank of America, as you can withdraw money from any Barclays ATM without fees, and you get great wholesale rates on the conversion. BoA accounts, if opened online, do not have any maintenance fees or balance requirements, so it is also a great option if you are just visiting the UK on vacation and don't want to mess with traveller's cheques.
Another bank that I really like is Ally (formerly GMAC). This bank is completely online, and they just started offering a checking account with no fees or balance requirements, and it pays interest. They absorb all ATM fees, but you will be charged 1% from Visa if you use your debit card to withdraw money in the UK. Another thing I like is you can link your savings account to your checking account so that it acts as an overdraft, and their savings rates are some of the best in the country.
These accounts can be seen as 'add-ons' to your main accounts. For instance, like many people on this board I have an account that my mom has access to that's with Wachovia that I use as my main account, but it's of no use in the UK, even if there were rumours of an international branch somewhere in London.