Glasses question (ok more Health than S&B but it'll get answered quicker here)

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I wear glasses IRL and vari-focals at work (because I'm old and I do close up work).

I want to buy some of those glasses that darken instantly in exposure to sunlight- what do I ask for?
I'd need just my regular prescription not the eyewateringly expensive varifocals though.

Where's the best place to look- I'm with Donald& Atchison (now taken over by Boots) but these kind of things could I buy elsewhere with my Prescription?
Do Costco do these? (They have an instore optician part)
Or anywhere online that's reputable?

They'd be for driving/being out and about in summer. I've got Prescription sunglasses but of course they are dark all the time so no good indoors.

TIA

Think they are reactolight, or reactolite, glasses.

I'm a specsavers girl myself.

Asda opticians are very reasonable

The general name is photochromic lenses. Things may have changed in the past few years but the ones I had a while back did not darken when driving the car because the glass filtered out the UV which is what triggers the lenses to darken

Transitions is the brand name for plastic lenses that darken in the sun. They are not guaranteed to darken behind a car windscreen due to the window blocking out the uv light which they need to darken.
You can get them online but you'll need your prescription and PD (pupil distance) measurement which is unlikely to be on there.
You can get lenses which darken behind a windscreen called drivewear but they start off quite dark anyway which looks odd and isnt practical and they're quite expensive.

Thanks-

I might pop into Boots and ask them (I'm sure they'll have some sort of special offer on sometime) but I wanted to know what to ask them for.

Stands to reason about the car windscreen, mine is a bit tinted. But when I get out of the car, the darkening would be ideal (and it doesn't hurt to have spare glasses).