IANAL

You don't know if the pump has to be manually approved prior to pumping though.

Hey mate, you may not notice since they're usually pretty quick. But all pumps in the UK have to be turned on and off manually by the guys running the petrol station. They're watching out the window and press a button. Feel free to give it a google if you don't believe me.

Also the law on petrol to under 16 year olds is based on supplying petrol in any form which was already done, paying for it makes no different at that point.

And as for the cachier rejecting payment because they falsely believed OP couldn't buy it. OP was trying to settle a debt at that point, and payment against a debt cannot be rejected without voiding the debt in the UK. So if they said no we won't let you pay he could've just walked out as long as intent to pay was clear. Leaving £5 on the counter would be the best way to do so of course, otherwise you may leave yourself open to scrupulation/investigation from the police despite no laws being broken. While a payment left on the counter clearly recorded on cctv leaves proof and dismisses the idea of theft immediately here for the reasons I stated above.