In reply to Tim Chappell vaguely but I got a bit carried away.
Sun glasses sold in the UK are supposed to comply with CE regulations. This states that the sunglasses actually do filter out UV radiation. Some available off the market stall or ebay, particularly fakes may have this sticker but not acually provide the protection, which can cause worse damage as the dark glass causes your irises to expand letting in more UV. This is a moot point though, and glasses bought at Boots or otherwise will be safe. What you pay higher for are the name, comfort, style, reliability and visual clarity. Obviously Name and Style are objective, and the name in partocular can bump the price up needlessly. Paying a little more can increase the comfort and reliability.
What can be important, particularly for all day use is the visual clarity. Just because the lens stop UV doesn't mean they are made that well. Cheaper lenses will often distort images towards the side of the lens. To test hold the sunglasses up in front of your eyes, about 10 cm away, look at a straight edge and then move the glasses around. A good pair should not bend the line in anyway.
In addition to the UV CE mark, some sunglasses are given a Cat rating. This specifies how much visible and IR light is allowed through. Cat 0 is clear and Cat 4 is Glacier/Mountain level, only allowing about 2% of visible light through. Cat 4 sunglasses are too dark for driving and probably for general use unless you have sensitive eyes. Most standard glasses are Cat3 with 'fashion' glasses often running at Cat2. Photochromic glasses will change their category rating depending on the light level, usally from Cat2-Cat4. This change is controlled by the UV so it won't work while driving as the windscreen will cut out the UV before it hits the glasses. Polarizing lenses cut out the horizontally polarized light, letting through only the vertical polarized light. This reduces glare from water, roads, snow and other flat surfaces. Obviously both photchromic and polarizing lenses cost more.
I've found that £30+ glasses from the likes of Bloc and Julbo suit me fine. I've bene buying Julbo for about 5 years and have been happy wiht all of them. Offhand I seem to remember their 'standard' glasses are generaly in the £40-£60 mark, the Polarizing seem to cost about £65 and the Photochromic ones £70-£85. Bloc seem to be a little cheaper. Something more expensive such as Oakleys supposedly have a better lens (although I can't notice the difference myself) and have better support (Oakley will supposedly replace your frame for free if it gets damaged).
