The money will go AWOL and you will be blamed. Even if you never gave your name, you will be tracked down at great public expense via CCTV and ATM cameras. The receipt you got will be deemed to be entirely invalid and proof of nothing.
It will get almost as far as court before someone senior enough to be capable of thinking objectively will look at it and realise it's all rubbish.
There you go, something along those lines will happen. It's almost guaranteed in jobsworth, incompetent Britain today.
P.S. How did the Tesco customer service desk react?