A UK bank account will make paying bills much less of a hassle, as they can just set up a direct debit on your account (which seems to be the norm here, much more than writing cheques.) Salary is usually paid straight into a bank account as well. It's also nice not to have to worry about currency conversion/the exchange rate/ATM fees when making transactions or using the ATM.
You can't transfer accounts, but it is possible to open a regular UK HSBC account from the US...I know a few people on here have done it successfully. (Seems to be a pain and rather complicated, though.) There's a thread on this board somewhere about that.
It can be such a pain to get an account (mostly due to the proof of address requirement, which can be a stumbling block for those newly arrived), but it's worth it in the end if you're going to be in the UK for more than a few months. HSBC's special Passport account seems to be easier to open since it's designed for expats, but if you've got proof of address you can just go into any bank and be able to open a account (theoretically anyway, haha.) You may not get a current account right away, but a starter "cash" account..but even that varies by bank and even by branch. Just shop around and try your luck along the high street...you might get lucky