Though the limitations of usng it are a pain, American Express usually has the best cashback deals (not even sure I could benefit now as I've used them several times before). At the moment I'm using Barclaycard Cashback and Lloyds Bank Premier, which aren't great and have absurdly low credit limits (I use them via Curve, so can switch).

Best source always in the UK is https://www.moneysavingexpert.com/credit-cards/best-credit-card-rewards/

Nothing compares to the annual free night in any IHG hotel plus tens of thousands of points with the IHG card - but then Creation had the habit of freezing my card for 'fraud risk' without telling me and because of some monthly standard transaction, etc etc... I took them to the Ombudsman over a dozen times I think (not that that did any good).