How to be a girl

The wonderful hadassah has generously offered to accompany me on a fact-finding mission to look for potential wedding dresses. Bear in mind that I want a bright-coloured party dress that I can wear for other occasions, not a traditional wedding dress or anything white. However I'm basically terrified of clothes shopping, and although hadassah is very good at it, she is also a foreigner and doesn't really know where the best places are.

So for UK based people, my question is where in London would be a good place to look? I'm looking for any recs at all, department stores, boutiques, little off-the-beaten-track indie or even vintage shops that stand out above the rest... If I have a day in London, where should be my top priority?

For everyone, do you have any advice about how to go about shopping for a serious dress? I'm basically intending to spend a few hundred pounds for something off-the-peg but nice, and then spend perhaps as much again getting it altered to fit me perfectly, and buying accessories including high-end underwear. I would really prefer something made out of natural materials and ethically manufactured if at all possible, but I think the former is easier to find out than the latter. Do you know of any designers / brands who are known to pay a decent wage for the people who manufacture their clothes?

Also my plan is to avoid mentioning the W-word, because I don't want to be charged a premium and I don't want to be steered towards stuff that looks wedding-y. I think I'll say "I'm going to reach the third of a century mark next year, and I'm planning a big party", both of which are true but aren't as connected as that would imply.