I use MSM on my older, retired, arthritic gelding, and it seems to be helping.
He'll even break into a canter once in awhile when the horses are going up to the barn for dinner. He hasn't done that in quite awhile.
I do find that the dosing instructions seem to be off, though. I have to give him the loading dose every day. The regular dose didn't appear to be working, but I started giving him the loading dose and it's made a world of difference.
I'm sure you can get MSM in the UK. It's not a controlled substance, and it's fairly inexpensive.