We expect the FX fee on GBP / USD transfers to be from 0.20%.
This fee applies only when you move money and not on every trade, which means once you've got USD you can trade commission free as much as you like.
Other brokers force you to convert back into GBP after every trade at a higher FX rate, so you get slugged with an FX fee on every trade. Don't settle for that!
If you want to bring or transfer USD yourself, you can always use your existing FX provider to fund your trading account.