Specsavers is a franchise type business model, so the stores are often individually owned. They sell only their own various brands of frames, but their range of contact lenses is probably no worse – and possibly better – than an independent’s since they offer a wide variety of main brand contact lenses repackaged under their own brands.
So I guess they would not be your first choice if you want a particular brand of frame, such as Oakley prescription glasses. Otherwise, I suspect Specsavers’ group requirements for the franchises and desire to protect their brand should mean that the stores are required to meet various service and quality standards, whereas independents may vary more, and it may not be easy to identify which independents are not good, and which are better than the chains.