In reply to Derry:
I've had an old pair of Bloc prescription shades with a fairly low tint for donkeys years. They're not Polarised, but have proven excellent for mountain biking and driving.
My prescription has eventually changed enough to buy a new pair, a Bolle frame. I got these Polarised and darker. To be honest, a lot of the time I prefer the lighter tint of the Blocs, but in the mountains on snow, the Bolles are a different league, and much of this is, I'm sure, due to the Polarisation. I'm also a paddler, and its nice to be able to see through the reflections on clear water as well! You will, though get enough protection form a dark pair of non-Polarised lenses, as long as they're up the UV block level. So, I'd say it depends how much you want to stick with Specsavers, if there's a good price deal there, I think you'll be OK, but will lose some performance. You'll be able to see your phone/camera screen more easily though, I can see bugger all on mine with the Bolles on!