Jeekers you’re rude
Yet you go on to say.....
Why not find some styles she likes online and get a dressmaker to make one?
Ask where her friends go for dresses.
Buy online and get altered.
An under £500 dress is always going to be an under £500 dress, regardless of where you get it from.
And didn't bother to answer the actual question!
The reason we are not getting a dressmaker to make one is because we don't want to do that. Not being rude, but it wasn't an option from the start of the thread so not sure why you think it should be.
She is fully aware of various plans of the girls in her year; still not relevant, we are looking to buy in London, so not fussed if her friends buy local or nearest city or whatever.
We don't want to buy online, hence asking for ideas about shops. Covered this aspect way back.
As for the '£500' dress comment! That's probably one of the rudest on the thread tbh.