Where is a good place to buy reasonably priced adult bridesmaid dresses in the UK?

I would search on Google .uk and territory.

The trick to not paying the 'wedding markup' is to buy dresses that aren't specifically for weddings. Find something off the rack you like that isn't outrageous and get a seamstress to do any fitting that's necessary.

Bridesmaid dresses don't have to be from bridal stores - you can look in any department stores (or in fact anywhere that sells dresses)

Try 'google'.. Lots of options

Don't buy in the bridesmaids section if you can. Of course that depends on the style/type of dress you're looking for.

Lots of the department stores now have a bridal section, Debenhams, house of Fraser, John Lewis. Next.

The UK is fairly vast. Might you narrow down your search radius a bit, along some indication of your price range?