I use Currencies Direct for transfers from Sterling to various currencies for payments. Their rates are better than any bank and generally competitive with the other forex dealers. Their website is good and you can payments on line.
The only problem I've found is that sometimes you'll convert GBP into a non-major currency (recently in my case for payments to China and Zambia) and even when you lock in the rate, the recipient will get less in the local currency than you wanted them to get, because of intermediary charges. I've taken this up with them but their answer is that intermediary charges are beyond their control and they have no way of knowing in advance when they will apply and how much they will be. They're not big amounts but it's annoying and time-wasting when you have to send a second amount to cover the shortage.