Originally Posted by NWIFlyer

Most major (chain) stores will accept AmEx these days. Because of the extra charges levied compared to Visa and MasterCard, the vast, vast majority of smaller outlets will not - which may well include pubs. If you're in London, the Underground and Transport for London buses will be fine, and outside that all train operators as well.

Whilst it's not something I've ever looked out for, my inclination is that Discover will not be recognised as widely - I certainly can't recall routinely seeing it on the stickers on shop doors, although don't take this as gospel!

All UK cards now incorporate Chip & Pin, whether credit or debit, and it's a much more secure and convenient way to pay. Contactless is being used more regularly as well if your cards have that feature. If not, you'll just be asked to sign - obviously every store still has the ability to produce a paper slip to sign.