I use a local one for my glasses and they've always been fine. Bizarrely though, I get my contacts through Dolland & Aitchison, who are great on that part of the service but their eye tests are bloody expensive! (£25 a time!)
Edit: I should say it won't really make much difference - the standard eye test is the same everywhere so as long as you have someone qualified doing it, it doesn't matter. You can still buy glasses and contacts from other opticians provided you have a printed copy of your prescription (I bought my last pair from Boots when they had a sale on)