Also you can get a lot of under informed people when showing them an American cheque, like it is some sort of strange device... Since it is your account and coming from your account, they can do it (and are supposed to do it). It will some times takes weeks for the funds to be available once deposited. You usually have better luck at the branch your account is at.
I don't know if you have a valid debit card from your US account, but you might be better off just withdrawing the cash on the card and then if you want to/need to deposit it in the UK account. You usually get a very good exchange rate on those type of withdrawls, and at least my US bank doesn't charge me too much for foreign withdrawls on top of the exchange rate.
You can also "wire" money from the one account to another if you need to move a larger sum of money. This usually costs money, like $50-$60 but usually only takes a couple of days at most.
Then of course there is XEtrade which is discussed on this board quite a lot.