My experience has been that the optometrists are very good and thorough, but that there is an increasingly obvious push by the dispensing staff to flog you stuff. Their lenses are pretty much like everyone else's in the same range - Vision express, Asda etc - all of these seem good with straightforward simple prescriptions but less able to offer you a top of the range thin flat lens if, like me, you seek to avoid the bottle-bottom look.
I always ask to see a pair of specs made up that has a similar strength to my prescription - this will give you an idea of an optician's quality of work. If they can't / won't do this, just have your eye test, ask for your prescription and go shop around.