When I transferred money a couple of months ago I looked at Xoom, Paypal, and Transferwise. On $3000, Transferwise gave me close to $100 more than Xoom, and even better against Paypal..
It was a bit of a pain to set up, partly because I use a small local credit union, but other than that it was fine. I would advise setting everything up a week or so before you need the cash in the UK to make sure it's ready when you need it.