Over glasses for prescription glasses

One option is a helmet with a built-in visor, such as the Giro Air Attack. Not all have enough room to fit prescription glasses underneath the visor, so try before you buy. The other major drawback, for me at least, is at these tend to be designed as aero helmets for time trials and the like, so it looks as though I should be going a lot faster than I am. 

I can recommend Optilabs, just off the Purley Way in Croydon (they're online too). They have a range of frames/tints with the prescription built into the lens, rather than goggles over glasses, or two layer glasses. (They do 2 layer glasses too if you prefer).

I would buy a suitable "cycling"/sports frame where the lenses are two separate pieces of glass (i.e. not a wraparound), then have an optician glaze it.

I have a pair of Oakley Racing Jackets glazed with glare-reducing, light-reacting glass - it's probably the best thing I've ever bought for riding. Contact lenses used to blow out of my eyes on descents. I carried different lenses for night, overcast and sunny conditions.