Maxi, elegant wedding guest dress, Central London

[Grumpy101]

Going to a very very fancy wedding with people I haven't seen in 10 years. I have 24 hours in London to shop beforehand (I live on a small island, buying online is not possible). Where do I go in Central London? Budget £500. I have looked at Reiss, Ba&sh etc but can't see anything.

Needs to be floor length so I don't have to wear heels (can't be bothered with them since the pandemic but I have chubby feet which look like stuffed piggies in anything other than nice heels).

Ideas where to go? I find Selfridges is too high end.

I have a lovely back up dress but it's a bit tight and I can't seem to shake the last 3 lbs.

Floor length, my go to is Ghost. Beautifully cut and elegant. I've worn their dresses as my own wedding dress. Twice! They're simple though not like a bride (I don't think anyway)

DVF
Rixo
Needle & Thread

I'd go some other department store (John Lewis?) that have many different brands, or for example Westfield.