I've used these people many times :
https://www.glassesdirect.co.uk/
I've had a very occasional pair that weren't quite right or needed tweaking and they were tweaked/exchanged/refunded without a quibble.
I would say that I have a (very) simple prescription for distance/driving - if I needed varifocal or high-index lenses I'd probably not risk an online supplier - just too difficult to get them exactly right.
If you are going to order, you'll need one measurement that is not part of your optical prescription - the Pupillary Distance (PD) - worth asking whoever does your eye test to measure it and tell you what it is, rather than trying to estimate/measure it yourself.