Not at all that complicated, especially when in a garage:
A £49 fill and a tendered £50 note, which the garage can't show is a counterfeit, means that you've provided legal tender for the debt incurred. It is entirely their problem, not yours. You are paying a debt, legally incurred with good money. Tell them to suck the petrol out if they're un-happy.
I have frequently taken cash abroad in the form of £50 notes as a protection for any loss of credit cards or otherwise electronic cockups. In about 10-15 such trips, I've never/ever had £50 notes refused when I've converted to local currency in various banks/post offices, instead of using my cards.
However, I've also drawn 500 Euro notes to pay tradesmen in Europe, worth £350 or there abouts. Again, no probs!
I think UK folks are a tad paranoid, there's lots of ways that traders can check validity of £50 notes. I think the problem is that in the UK, traders can't be arsed so it's easier to inconvience lots of other people and reject notes.
So; Now I plan to draw good £50s from my bank, fill to £49 at my local petrol stations, offer genuine notes straight out of Barclays and drive off if refused, f' 'em EP