Disregarding government data aside, it really isn't a money gimmick, it's a deterrent. The article says the fine will be around $70, so I doubt they get more than a few thousand dollars. (Or pound since this is UK)
It's a form of discretionary weapon. There are far more laws on the books than any law enforcement agency can actually enforce, and the nature of many of those laws make them next to impossible to enforce outside of invasive procedures or 'citizen snitch' reporting.
This always causes such laws to be used as weapons. Political enemies become primary targets for such laws, or neighbor feuds end up involving reports to law enforcement (true or not) regarding infractions.
Generally speaking, however, these types of fines are 'add-ons.' Someone gets pulled over for a bit of speeding, then dinged for not wearing a seatbelt, then hit for smoking in the car with a kid, then hit for wearing last year's fashion trend, and whatever else.
I'm having difficulty generating traffic stop statistics for the UK, but according to the wiki:
" Between 1 December 2007 and 30 November 2008, traffic officers patrolled 2,025 miles (3,259 km) of (Highways England managed) motorways and trunk roads across England, every hour of every day and attended over 281,000 incidents, an average of one incident every two minutes.[5] Due to government budget cuts in 2013, traffic officers no longer patrol the network. They are instructed to be reactive and only attend reported incidents. "
Frankly - they don't even patrol any longer, so the law may as well not even exist as the only way it will be enforced is if someone reports it... or if it is added on to another response. Wreck and smoked in the car with your kid? There's a fine for that.
Even so, if we assume just 5% of the incidents would result in a hit for the fine and the number of incident responses is remotely similar to 280,000 - that's 14,000 incidents that can hit upon a fine of $70 - which is an estimated revenue of $980,000 on incidents that would be responded to, anyway and contribute to officer time.
Further, many of these types of laws have "second infraction" additional fines. Similar laws that have been enacted in States and municipalities in the U.S. may be an initial fine of $20 but then elevate to over $100 for second infractions.
Generally speaking, they aren't an effective deterrent. They are simply an added fee they can cache in on when they are responding to another issue. Just like seat belt laws.