i was the reverse situation a few months ago when i needed to transfer $ to £ for the down payment on my UK house. i was in a loss at what to do, as I usually just write a check in US dollars and deposit it in my UK bank. It takes 2 weeks or so to clear and there's a fee from the UK bank.
After looking around for a reputable currency exchange company, I used this company:
http://www.customhouse.com/ (their UK Branch)
I choose them for two reasons:
1- My sister used them to transfer $ to Canadian $. She was happy with their service.
2- They had the most competitive buy/sell difference. I called my US bank and asked them hypothetically if I were to transfer $50,000 today, what would the buy/sell difference be. Using my US bank, I would have gotten less pounds.
However, it was one more step in the house buying process. I had to first register with them, filling in a form and sending it back. Then I had to wait until the exchange rate was ideal. Next I transferred money from my US bank to their US bank, they then transferred the money to their UK bank, and finally into my UK bank. The whole process took 2 working days.