Typically the bride and groom pay for the BM dresses and mens suits as you are usually asking people to wear something that you have chosen. If you are happy to have BMs in any dress for instance as long as it’s say pink and full length for example then they would cover this cost their selves. In the UK it is usually the couple providing the outfits and sometimes BMs can buy the shoes and possibly wear them again.
I offered to pay for my bridesmaids’ dresses and let them have a lot of say in what they wanted. I gave them the budget and ideas of what I don’t particularly like in bridesmaid dresses. The caveat I gave them though was that if they chose dresses over the budget, they paid the difference. Happily they all liked a dress style I found which was about £35 under budget/dress (they chose the colours and the exact style) 😁
I'm doing the same thing as Georgie! It's definitely expected in the UK that you pay, but you can absolutely set a budget and ask them to pay the difference if they all fall in love with something over budget and would rather wear that.
Similarly for the guys suits, we're planning on renting these but if anyone wants to own theirs they can pay the difference to buy.