There are good and bad examples of both. However independent opticians live or die on their reputation, whereas chain branches get custom just because people recognise the brand name. A longstanding independent optician, especially in a location with competition from Specsavers and similar, is likely to be a good one.

I use an award-winning independent for my main glasses, and have back up pairs from Specsavers. The latter are cheaper, but the quality of both frames and lenses, while adequate, is noticeably worse. Also, as you say, the customer service is not on the same level.

My 'good' glasses cost around 4x as much as my Specsavers ones, and while they're better, they're not 4x better. Whether you think a good independent is worth the extra expense is a personal decision.