There's a lot of Brits importing old sports cars from the States still, and there are shipping companies regularly bringing containers across with about 4 - 6 cars per container. I have previously shipped a set of alloy wheels with tyres (crated) across from a shipping company in Carolina in a container that was coming across. It helped that one of the cars in the container belonged to a friend, but I managed to pay only about $100 or so for a dark corner of a container. They got across OK, but it was obvious someone had jemmied them open at one point.

Try looking up companies specialising in car imports in the US and calling to see if they'd be willing to do this.