No idea!
But... There's a register on the samba.com which you could trawl down and pick out all the UK zwitters. I can't remember whether it lists LHD/RHD though. It might not be very up to date...
I'm yet to meet another zwitter, LHD or RHD, but I know there are a few around. Several people have come up to me at Shows who are restoring RHD zwitters and want to check out what parts are on my LHD. This usually sparks a fun debate with the conclusion that no one knows for certain, but demonstrates there are some out there.
No doubt others can offer a more accurate answer!