I never use sunglasses Dave. But I do buy my glasses (for myopia) at our large Tesco branch. The last ones only cost £20, including the lenses to my prescription. And all their eye tests are free. I don't reckon one could get anything cheaper than that online. Might be just as good value for sunglasses.

But I see that Which and MoneySavingExpert have reviews of online purchases.

I like to have my new specs properly fitted and adjusted by the optician's assistant at Tesco. They sometimes need tightening or melting and bending in that hairdryer thing.