Given the price at the register is $X across all payment methods, I prefer to use my card that lets me pay 0.96X 30 days later. (I don't know what discount you can get from UK rewards cards...)

I hope that rewards cards go away, because they distort the market, but I'm going to use them while they're here. Rewards cards push costs onto customers that don't use them, and I don't want costs pushed onto me.