Are you looking to hold the pounds, or are you looking to convert?
I use OFX for euro to USD, and have been happy with them. I second the observation that they are very sensitive to money laundering, and may limit or delay your first transactions, until you have established a pattern.
If you wanted to hold it, you might also check out Interactive Brokers. They don't want to be your bank, they are a broker. But that means you might be able to put the money to work faster than trickling in foreign cash transfers.