You don't have to though, you can wear something you already have if you want. Making someone pay £250 for a dress plus shoes and hair that they have no choice on isn't fair IMO. For me, I do not want anyone to be out of pocket for something I am organising, I have stressed to everyone please do not buy a new outfit if you don't want etc.But you have to get a dress even if you're just a regular guest? Don't get me wrong, paying £250 for a dress you didn't choose yourself and won't wear again is way too much, but I'll be maid of honour to my best friend in may and I ordered my dress from Asos (and I chose it myself, no dress code or colour scheme), it was around £40 and I'll probably be able to wear again. She's also my MOH in july and she bought her dress that she chose in Zara for 50e.
If you can pick your dress and/or you have been asked if its okay and agreed etc then that's different, but forcing it on people is not okay.