In North-East England there is a shortage of locum optom’s so rates are higher, Tesco often offer £300-£400 if they have no one available. Although the test room are an ergonomic slum to work in.
Other multiples are getting the message and raising the rates. The old capitalist supply and demand I suppose. Specsavers were terrible for booking you and then offering a lower rate a few years ago, so people stopped working for them, and now they are better behaved, but they will still try their best to get a reduced rate out of you.
Independents tend to pay higher rates and as long as you show loyalty to them and a concern for their business, they will be good to you in the long run.