I looked into the legality of fitting T rated tyres (up to 119mph from memory) to a TR6 originally fitted with H rated tyres. In the UK there is no obligation to fit tyres capable of the car's top speed but in some countries (as stated by Jean R above) you must fit original specification or better. So not an MOT issue or insurance as long as you don't get caught doing more than 119mph. If you do, the type of tyres you are using will be the least of your worries!
Having said all that, I still think 185/70 R15 is the optimum size for 5.5J steel wheels and they are really hard to find. Pirelli were listing P6000s in this size but I think this was a mistake as I now can't find them. My car has period style upgrades to brakes, suspension and engine but looks standard. 185/70 tyres suit the ethos of my particular car and I suspect many others.