Does the UK have above average just-shy-of-two-years turnover? Seems there might be a small incentive to let people go before that threshold.
It's similar to probationary periods - you'll see a few people fired right at the three month mark but it's usually more of a case of giving that employee grace for the probationary period rather than accelerating a firing that'd happen later.
"Section 94 does not apply to the dismissal of an employee unless he has been continuously employed for a period of not less than two years ending with the effective date of termination"
Section 94 is the name of a section of the current law regarding the general idea of like, employing people, and that section says you can't unfairly dismiss them, so this sub-sub clause says that only applies after two years.
Edited to add: This stuff is easier to find in the UK because although it's legalese, the Westminster Parliament is obliged by its own rules to name laws after what they do, this is an employment law so I searched for "Employment". In the US it might be called the NICEHAT Act or Jim Smith's Law or some other useless nonsense because they have no such rule.
Persuading the tax man that the cleaners who in effect are your direct employees are "freelance" somehow and so you shouldn't have to pay the tax. Or claiming that the labourers you hired don't get paid from 0830 because they aren't "really" working until 0900 even though they're in a company minibus being driven to the worksite and this is the only way they could get there.
Neither of those things is legal here but both of them might happen and will only sometimes get reported and those responsible will get at worst a fine. If you tried anything like this shit with Sam who has a penthouse office in the City he'll summon a swarm of employment lawyers and ruin your life, but because Mateusz who cleans the toilets has dubious immigration status and he's behind on his rent he's probably not going to make a fuss when you steal from him this way. This sucks.
The problems are always at the very bottom. People who are closest to the poverty line, most easily replaceable, least easily able to organize and demand better and also most separated from those who have more freedom to demand change. One important angle unions think about is schemes to separate these low wages workers from colleagues who would be able to advocate for improvements. If the guy cleaning the toilets doesn't legally work for the same company as the guy who uses them, suddenly the question of whether "cleaning toilets" pays a living wage isn't a matter of employee solidarity...
Edited: Swapped Manhattan for the City (of London) because the topic is specifically UK employment