As a former optical technician now in the airlines, but not fully conversent on the CAA rules) If you have a high prescription then you could get prescriptions on an refractive index of 1.74 for plastic and 1.9 for glass.
If your shortsighted and want photochromaticsn that are an even colour across the lens then you need to ask about Carl Zeiss equitint. If you want a solid tint then try essilor and their products.
Sorry if thwe info above is out of date but it is five years since I left to play with the big stuff.
Talk to your optician, both opthalmic and dispencing. to assure correct fitting of bif's or vari's take him to the plane and both sit in it whilst he measures you up. NO snide remarks please
have fun and have a bigger wallet
Plastic