Nah, not even close. The rims are what, $60 / piece?

5 lbs. from the US to the UK via USPS would cost $33.75. If it was just the rims, that's not too bad, IMHO. It might be a little more than that with handling and it would def. cost more if you purchased a wheelset, but I wouldn't call it prohibitive.

Maybe you could even convince them to mark it as a gift with a low declared value to save on the customs....