CAA advice on sunglasses including prescription lenses here
Guidance on using sunglasses | Medical | Safety Regulation
Interesting points in here are that you can have a pair of prescriptions sunglasses as your second pair. Also that they don't like you putting non-prescription sunglasses over your normal glasses. Not sure if this applies to clip-ons.
With the CAA advice in mind, I think the best answer is to use an optician with a 2 for 1 deal and get a set of normal specs and sunglasses made up to the same prescription. I had a simple grey tint put on mine and they work just fine, especially in those low sun landings in winter. The only downside with a high prescription is that my optician couldn't supply sunglasses lenses a with high refractive index, so they ended up being quite thick.