I have a US$ bank account with Citibank here in the UK, I also have one with Bank Of America in Florida. The one in Florida was because when I had a house out there it made the transactions a lot easier, but I opened the one in London because moving money out of the US while in the UK was nigh on impossible.
The Citibank account has been pretty useful, I have a debit card (with a visa logo) on it to use in shops/restaurants etc and an ATM card for drawing money out (either in the US or from my branch in London). If I hadn't had the house in the US or wasn't planning to go back over to live I'd have closed the BOA account down as they require you to personally visit the branch for a number of transactions that are impractical if you're living in the UK.
As you're from Barking it shouldn't be too much of a journey to get into London, I use the branch that is just along from Monument Tube station towards the Tower, I think it's in Great Tower Street. There is also a branch near Covent Garden.