iPhone 12 mini 64GB is currently £26 per month on Vodafone with £75 up front via USwitch. Total cost comes to £699, same as the RRP of the phone itself. Best price I think I've seen for this new.
Not sure whether there's any further possibility of cashback. There is the potential of using Mobiles.co.uk
directly for the same contract with £79 up front and getting cashback from them, but I've heard they're unreliable in honouring any cash back incentives.