They won't/shouldn't convert for you. They'll let your bank do it(with whatever conversion fees, if your bank has them). It's probably just a graphical error where a $ is showing instead of a £. Whenever you make enough to get a payout, you'll be paid in £ and it'll go into your bank as $.
Or I assume that's how it is, since it makes sense. I don't think anyone's actually made enough through UK sales on PubIt! to have a payout from them yet, but they -should- be separate storefronts, like Amazon and others, because otherwise it's a huge accounting nightmare and a million people will complain(PubIt! users, and people that work at B&N).
I wouldn't worry about it too much until the time comes to get paid for it.
Also, I just double checked, and they don't actually even put a £ or $ at the bottom for the monetary amounts, so I doubly wouldn't worry about it. They have them in the individual sales, for whatever reason, but yeah... probably just a mistake. They shouldn't/wouldn't want to immediately convert £ to $ because if they did that individually for every sale it would become very expensive for them.