Currency Fair or a similar company if you have a bank account in both currencies. I've found that sometimes the best way to get the best rate with them is convert in the lots you see open, they list from the best rate down, so if you do it all at once, you might not get the best conversion available. The pound is quite strong at the minute too, so bear that in mind.
Edit: I should have said, you can transfer the full amount of dollars in one go and then do the GBP exchange in pieces as you see fit.
https://www.currencyfair.com