I just thought of this while thinking of getting a topup of my electricity prepay meter. I'm sure most people know about the Amex Small Business offer valid 6th to 21st Dec. You register your Amex card on the small business website, then spent £10 on your Amex at a small business local that is on the approved list of Amex, you get £5 back as statement credit.
My local off license does Prepay topups, accepts Amex, and is on the approved list. So I get a £10 topup for electricity, and get £5 back. Then I go find the next off license that does the same, and another £10 topup. Then I do it again, and again, and again. In no time I have a few months of electricity supply for half the price!
Tell that to the Big 6!