I would have a pump nozzle with a locking collar and matching fitting on the car. The collar would engage with a half twist, and would not dispense without being locked in place. Likewise it would need to be locked back on to the dispensing pump after use, which could be monitored by the kiosk attendant to prevent accidental damage from driving off.
But the good bit comes from a clockwise fitting for petrol and anticlock for derv.
It sounds more fiddly than it actually is. Just ask anyone who has owned an LPG car.
I have a diesel car, my wife a petrol one; I have a small garden diesel tractor and a petrol mower. I have also mistakenly put petrol in my car in a moment of inattention. Cost quite a bit to get it out etc. My local breakdown people say they get about 10 a week!
and drove it from Heathrow to Guildford. I was young, it was late at night, and a company car, and I was simply concerned with getting home. The car ran beautifully, although it smoked like a factory chimney. The next day a local garage flushed the tank, and I drove the car for another year without any apparent ill-effect.
Engines were a lot less sophisticated then - I can imagine the catastrophe if I tried it with my current car.
The Essex Police solution makes good sense to me, and I'm surprised nobody thought of it before.