I would not be so complacent. I agree it's unusual for a large retailer not to accept Amex, but, outside London, IME it's rare for a small retailer to take it. Even in London I have only a 50% success rate. Just last night at the theatre, they wouldn't take Amex for the intermission drinks. You absolutely need another card and I would go further and suggest having back up cash for very remote parts of Cumbria.