I am just ending my time on the BUNAC visa, and I brought over my money in a combination of cash and travellers' cheques from American Express. If you happen to have an Amex card (or possibly someone in your family) you can get the travellers cheques in pounds with no fee. There was no problem in depositing them at the branch of Natwest BUNAC uses. I got a good rate, slightly worse than if I had done a wire transfer, but I wanted to be able to immediately deposit money into my account and have access to it.
Keep in mind that to enter the UK on the BUNAC visa you will need to have immediate access to 1000 pounds, and if you don't have housing, etc arranged, you will need that money for a deposit! Depending on if BUNAC and Natwest are busy, it can be a couple days wait for the appointment for the account to be opened. Then add on the time it takes for the money to transfer and clear into your account - another few days minimum. You really don't want all of your money to be tied up in that way. (in limbo between the two accounts!) So if you decide to transfer some of your money over, make sure you bring over a minimum of 1000 pounds, or can easily pull out cash from ATMs in the UK without your bank charging a hefty fee. They can in fact quiz you at immigration about whether or not you have access to that money as a condition of your entry. I wasn't quizzed, but knew of several people who were.
If you have any other questions about BUNAC, let me know.

It has been a great experience and I plan on repeating it after my graduation next year!