Yea it will vary from place to place, but it's always best to have a spare untinted set for MOT time, they can be quite strict about these things. There is room for plenty of speculation here, lots of people say as long as the reflectors are still, well, reflecting and you can still see brakes, indicators and fog you should be fine, but if they want to fail you on it they could- because you can't really prove that they are not blocking out too much light.
I put standard lights on mine when it goes in for MOT, just to be safe, but I've never been pulled for my lights before.
Bottom line is, if you put your car in for MOT with tinted tails, your taking a chance.