IME it's a bit of an urban myth that AmEx is not widely accepted in the UK.

Just because there's no blue badge on the door, or because once, way back in the mists of time a merchant didn't accept it, doesn't mean that they don't now.

Many businesses have recently signed new credit card 'acquiral' contracts and these include reduced fees for AmEx acceptance and, in some cases, sign-up incentives for the business.

So, I always ask, even if I know that a merchant hasn't historically taken the card. I accept that VISA/MC has and probably always will have a larger gross number of accepters in the UK, but AmEx is far from being a lame duck in this regard.