As far as I know, here in the US, the best deal you're going to get from a bank is the current conversion rate - plus you'll pay a fee depending on the currency and if you are a bank customer or not. If you can convert at the current rate without a fee at the post office - that's the best deal you're going to find. Even if it was 5% below exchange rate - it would be much cheaper than shipping the pounds over here, having them converted into greenback with a 10%-15% conversion fee, and shipped back.
Check the currency converter site at CNN that LadyCook posted and then compare it to what your post office conversion rate is for that day.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain