Hi Kal,
They give you their acct number and sort code during the transfer process. The acct number is different depending on what type of transfer you are doing, so make sure you write down the right number.
You have to get the money to them within 3 days, so as long as you complete the transfer to their bank right after you initiate the trade on XE Trade, you won't have any problems.
My final suggestion would be to set up an account and do a small transfer now (maybe £50), so that you can get comfortable with the process. Don't worry if the exchange rate is not fantastic, but the few pennies you will lose willl be worth it I think. This way, when you MUSt do a transfer, you should hopefully have less stress!