Unsure about what exactly the market is in the UK, but this post reminds me of a old /fit thread. Just go to any discount clothing retailer and grab a few pairs of "good enough" shorts, shirts, etc. You don't need to do much research if you're just looking to get into basic regular exercise. I think a lot of people experience a sort of analysis paralysis when theyre just getting started with working out and theyve never had to consider what clothes to wear. As long as you're not showing up in jeans and boots, you'll be fine; I workout in a pair of old jogger sweatpants I got a few Christmases ago, some old sneakers, and whatever old band shirt I pull out of the closet. What matters is that you get out there!
If you experience any catastrophic failures in your clothes, then you'll know what the weak points are, but odds are, you'll probably find that the cheap stuff can handle quite a bit of use.