I recently moved two suitcases worth of stuff over to the UK (30 mins north of London) from the US (Texas), and after checking the cheapest/slowest options from FedEx, UPS, and DHL, it was still considerably cheaper to just pay British Airways for the extra baggage.
I realize you're going the other way, but with such a small amount (relatively) it wouldn't make sense to go with one of the normal cheap shippers like UPakWeShip.
Hopefully someone will chime in with a better alternative, but I think the extra baggage will probably be your best bet.