It's looking a bit doubtful. I'm pretty good with the ol' Google, and I haven't been able to find one.

Even if you decide to go with the US tow bar, there are a couple of obstacles.......

We don't use bolt-on tow balls here. We use a detachable draw bar that takes a threaded-post type ball. You also won't be able to use a US tow ball, because ours are generally either 1-7/8" or 2", which is not 50mm.

On the plus side, your side of the pond you can get a 50mm tow ball with a 19mm threaded shaft, which is close enough to being the right size to fit the 3/4" hole you'll find on a US draw bar. I use this type of setup for the draw bar I use with the Focus and the Mini trailer (which has a UK coupling). I don't know how it works over there for heavier weighted trailers and caravans, but if you need to go with a 22mm or 25mm threaded ball you do generally have plenty of meat on a US draw bar to be able to open up the hole some.

Or you could fab-up a custom draw bar to take a bolt-on tow ball. I dare say that welding a length of 50mm X 10mm (or thereabouts, I don't know the specs of UK metals) steel bar (edit: properly gusseted, of course) to the end of it and drilling holes to match those on the tow ball would do the trick.