I've said it before but I'll never stop. Never, under any circumstances, buy dodgy cheap sunglasses. Buy them from somewhere trustworthy who might have a clue of their origin rather than from, say, market traders.
If they are cheap knock offs, the chance is that they just have dark glass in them with no UV protection. Because they are dark, your pupils open up and the UV floods in. Result in a few years is cataracts.
I'm not saying you need to go expensive, but you do need to buy them from someone with some credibility.
Mine are prescription, so well over £100, unhappily.
Vin