MIST is currently needed for guys working from Platforms, Jack-ups or MODU's in the North sea. I know of one Sat boat who had to get everyone 'misty' for a particular contract, but it's not widespread on vessels yet.
A lot of GOM companies use SafeGulf, but they should pay for it if they need you to have it.
I was in the North Sea last summer and all that was needed was a Dutch ticket. They looked over my log books and asked a few questions. I had to get some more recent dives logged, but after I did that they gave me the OK.
The problem in the North Sea is immigration. They have a lot of divers and it is so much cheaper and easier to hire local.