Bridesmaids / Groomsmen attire

You almost definitely should be paying for the wedding party's outfits - it's very much the expected custom here in the UK. However, it's totally up to you to determine how much you want to spend on these so don't feel pressured into going for things that feel ridiculously expensive - if the bridesmaid's all want a fancier dress you're well within your rights to say "I've only budgeted X for the dress, so if you all really want that one could you pay the difference? If not we'll need to find something else that's in my budget." Similar with groomsmen's suits, you can hire these and tell the guys that if they want to own them they can pay the difference.

If they want to offer to pay for the full thing that's really generous of them but make sure they are all in agreement so that no one feels pressured to pay for something they can't afford.