How to commit the perfect crime: what you need is a bank account with at least a hundred quid in it, a cash card for said account, and a big fuck-off sombrero.
You don the sombrero, making sure it's pulled down low at the front so your face can't be seen from above. Approach a cash point that's not in use and stick the card in, then request a £100 withdrawal. The machine will proffer five crisp twenties in a neat little stack, at which point you carefully peel the outer notes apart and extract the middle one, being very careful not to fully pull the others out of the slot. Wait around for a minute or so, and the machine will decide you must have changed your mind or something, and retract (most of) the money, re-crediting your account with £100 leaving you with £20 in your hand.
And the best part is, if the money is missed and someone asks you about it, your face won't be visible on the CCTV recording and you can just blame it on the Mexican who stole your cash card.