Thank you for the heads up. I hope you can find satisfaction at a different supplier. When examining you for your prescription, Opticians often ask to see your present spectacles as a starting point. Presumably, this means they can identify the actual lens refractiveness - in other words, the prescription for which they would have been appropriate. After your eye test, they can then tell you what is the true prescription for your eyes. Any difference could form the basis for a claim against Specsavers.
Another problem with Specsavers is the paucity of their Sunglasses range. I bought some prescription sunglasses from Specsavers. The correction is fine, but there is a problem in bright glaring sunlight, in that they do not "wrap around" a sufficient part of the face. In bright sunlight, I'm annoyingly aware of strong light beaming down over the top of the glasses, unfitered directly into my eyes, unless I wear a hat, which I don't always want to do. My non-prescription sunglasses (£3 a few years ago in Asda) protect my eyes perfectly from any stray bright-light and remove this discomfort. I went back to Specsavers to look again at their (small) Sunglass-Frames range - which looks pretty much the same as their standard Frames range - and tried all of them to see if they could match the Asda glasses for protection. None of them could, from the Brighton & Seaford branches. My wife has a similar problem with her sunglasses - she likes to read on holiday and finds that excess bright light comes in from the side.