I'm rather against such concessions because, while it saves time and money, it puts the police/enforcement officer in a position to be judge. If he's having a bad day and you don't suck his dick just right, you get fined, sometimes falsely. And now there's no "fair" court[1] to contest the fine in.
[1] Already, traffic courts are notorious for siding with the police in cases without any evidence for either side.
Edit: Speaking from a different perspective/experience. Maybe the UK courts are different than Canada's.
It is opt in though. If I didn't pay for my train ticket and got fined, I would much rather do it online than go to court. If it was something ridiculous like being caught fishing without having a license on me, but I actually did have a license, I wouldn't opt in and I would go to court instead.
It only becomes a problem when people don't have the option to contest it in court.
edit: I do think this is a good idea, but maybe it wouldn't even be considered if our police force wasn't vastly underfunded.
Here, you don't have to go to court if you're just paying the fine. You might have to pay at the courthouse but you don't have to plead guilty in front of a judge. It's already opt in.
It can also be a problem if people are persuaded that it's not worth fighting a bad ticket, for example if the total cost is higher when you contest the ticket (court fees).
Apart from the algorithmic determination of the fine, it sounds much like the system of "administrative punishments" in place here. Which is, unfortunately, biased against you by presumption of guilt and further biased because hiring a lawyer for a proper defense would cost as much as the fine.
If it is just stuff like not paying for train tickets and fishing licenses, I'd rather do this than have to go to court.