I've done it before and I think at least one other UKVAC'er has done it before also. In my case it was back in 2001. The way that it worked is that over the previous year or so UK folks would have their US purchases shipped to me in the US. They accumulated in my garage in the US with a plan that the whole lot, 24 or so games plus boxes of parts etc, would go in a 20ft container. The UKVAC folks planned to attend CalEx in September 2001 and the container was booked to load during their stay.
As it turned out, the world changed on Sept 11th and only one UKVAC'er got out before air was shut down. Despite that little bump, me the VAC'er, the truck driver and my dad managed to get all the stuff on the container and off it went. It arrived in the UK OK and a few of the UKVAC'ers managed the drop-off, collection and distribution of the contents accross the UK.
So it's doable, but not for the faint of heart...