If anyone has high index prescription glasses and doesn't or can't wear contacts, any recommendations re. sunglasses protection for road cycling in bright and low light conditions?
Normal prescription glasses and sunglasses (let alone any designed for sport) are very expensive when high index for -12.5 and such a prescription also precludes curved lenses.
It sounds like you need specs with a prescription insert (rather than putting the prescription in the coloured bit). That will also make updating your prescription cheaper in future, because it will only be a "plain" lens that needs changing, rather than the fancy bits.
If you were in Sheffield I'd suggest that you go to Eye Eye opticians https://www.eyeyesheffield.co.uk/ It may be worth contacting them anyway, to see if they can sort you out remotely, because they're super helpful and like to make sure that people get the right thing. They don't do pushy sales, and if they can't help you then they'll say so.
The only option for my -10 prescription was safety glasses, I look like a dork but they at least provide more protection than my regular glasses due to the side shields. I'm going to ask if my optician will glaze a pair with tinted lenses for next summer.
I tried inserts once, d they misted up horribly and the lenses were so small it was like looking through a tunnel.
I use a pair of Adidas Evil Eye with prescription inserts - I've had sunglasses for about 6 years and changed the inserts when my prescription changed a couple of years ago. They work really well - no misting up.
If you're not too bothered about looks you could try Cocoons overglasses (https://cocoons.uk/). I have a pair with the yellow polarised lenses and they're great in just about any UK conditions. Not many poseur points though. Good customer service too - I emailed them for replacement lens a few years back (only available in the US) and they sent some over with no fuss and no postage surcharge.