Mark,
Assuming it is a Big 6H, not a15/6H, the answer is probably less than the fingers (excluding thumbs) on one hand. I say probably because not all vehicle details are recorded on the membership list.
I know a Dutch member who did a well documented restoration of a 15/6H some years ago, including the conversion to LHM. If he does not pick up on your post soon, I shall attempt to draw it to his attention.
There are other 6Hs in the UK but I don’t know which fluid they are using. With luck, anybody who is using LHM will see this and reply.
In the meantime, I think it might be worth trawling the interweb because I have a feeling my Dutch friend had uploaded some useful information in the past.
B…..