Just to add to the locations, there is/was (last time I went anyway in 2001) a Sunglasses Hut at Crossroads. I have bought twice from their in the past, price around dollar per pound. Fairly knowledgable, but I did research first and knew exactly which pair to buy. Also, don't be tempted at the airport on the way, you will definately get that pair in Florida and much cheaper. They may also have a wider selection than in UK anyhow. I haven't noticed much difference in price from shop to shop, really all around $10-15 price range either way.
They will try to sell you lens cleaner, which they claim to be the only type that works with Oakley lenses. Don't be taken in if you don't want it - regular lens cleaner is just the same. You will pay more for it in these shops than at an opticians.