Bridesmaid dresses in central London

I’m coming to London in June from the Netherlands. I’m going to be a bridesmaid for a friend and since there’s not really any bridesmaid appropriate dress shops near me in the Netherlands. I was hoping to do some shopping for it in London. I’ve had a hard time finding shops though, does anyone have any suggestions? Budget is up to 600 pounds so I think maybe Harrods would be out of budget.

id really appreciate some help, thanks in advance.