First, I'm not a lawyer.
This bridesmaid-zilla has conned the OP out of £300 for a dress she wanted to wear to a third party's wedding, by the deception of telling the OP that it was for the OP's wedding. The OP's wedding is why the £300 was paid, and that amount only after considerable pressure from bridesmaid-zilla, obviously with a purpose behind it.
Am I the only one to think there's some criminal behaviour in this and far from letting the perp off the hook, the OP should be talking to the police about fraud, it's obtaining something of value by deception?
The £300 was paid for the bridesmaid to have the dress and wear it as a bridesmaid at the OP's wedding, and was the bridesmaid's to do with what she wanted AFTER the wedding, not before.
It's not any old dress, it's for one purpose only, the OP's wedding, and now it's secondhand and not returnable, and very likely to embarrass the OP if worn by this bridesmaid at her wedding. The £300 is a total loss to the OP; you won't be getting that back, OP. The other two dresses are also useless and have to be returned and replaced.
It's money, ffs, the legal system takes money offences very seriously.
Even more importantly, this will hang over the biggest day of your life, that you want to be perfect, and tarnish all your memories of it. Why should that be allowed to happen? Have a think about this OP, then tell this person she either pays the money back, you cancel her wedding invitation, cut her out of your life, and say nothing more about it; or you'll look into whether it's a crime and what to do about that. Why are you being so generous to someone who doesn't deserve it? Did she suddenly become this awful person after many years of beautiful friendship, or did you just not see it previously?
It's not about a dress, it's about being deliberately deceived by someone you trusted. Was it the first time she's ever done something nasty to a friend? No, I don't think so either.