AH Heliocpters at Dunkeswell in Devon operate 500s, training and fire fighting I believe, they have a good reputation.
The owner posts on here I am sure he will add on what else they do shortly.
I think Heli-Flight Gloucester have it on their training approval, don't know if they've got one.
Eastern Atlantic at Shoreham are the import agents for MD and always have a couple up for sale.
Training wise Dennis Kenyon , Andrew from AH, or Ray Jones (might be bob cant' remember) are the guys to go to if I recall correctly, I have no doubt missed someone out sorry.
Several in private hands, don't know a lot about them.