The course is about the same as an IMC (assuming you did that IMC in the UK - and not over here).
Few more hours, of course, but not more to it.
Cost, I have no idea...e-mail a few schools - or look on their websites.
Value in the UK?? Depends on what you want to fly. If you fly G reg aircraft then it will give you IMC rating privileges - which you already have. Nothing else.
If you fly N reg aircraft, you can fly it in any airway anywhere in the World.
If you do not have access to an N reg aircraft - it is probably worthless (to you).