this has not been my experience at all. i see them supported everywhere in london. and even supported in cornwall recently.
I’ve never noticed it, but I’m admittedly speedy when I travel. Maybe the high-end spots keep it while lower-end ones are walking away? Makes sense with e.g. eBay.
Realistically, Amex works in most metros in the UK I've travelled in. Maybe the local kebab shop doesn't accept it, but the overlap in customer base probably isn't there anyhow.
Citation needed.
It's a global financial centre and the central economic engine of the 6th biggest economy.