I wish the US had something like Art. 15 of the German Constitution. As for the UK, we are lucky in this regard with the flexibility our lack of rigid constitution offers

Fair enough, it does happen. Relevant background: https://nationalmortgageprofessional.com/news/investors-hold-most-empty-houses

The states with the highest vacancy rates in investor-owned properties were:

Indiana (7.2%); 
Illinois (6.1%); 
Oklahoma (5.9%); 
Alabama (5.9%); and 
Ohio (5.8%). 

States with the lowest vacancy rates were:

New Hampshire (0.9%); 
Vermont (1%); 
Idaho (1.2%); 
Utah (1.5%); and 
North Dakota (1.5%).

I'd argue my sentiment remains, investors for the most part rent out property, they don't typically hold it vacant. When they do it's more likely vacancies are transitory (trying to find a renter) than long term and intentionally kept vacant.

As well as holding onto housing stock to maintain a local monopoly.

It isn't as big a deal when you know any potential renter will be renting from you regardless of it is this unit or that unit. They are still forced to be your customer and you don't have to compete with lower prices from another landlord.

I think you're thinking of a case in the USA that colluded on rental prices? I don't know of any evidence that there is collusion on house prices in the UK. People separately talk about investment companies buying up housing in the US as if that will have much impact on prices, but the proportion of housing under discussion is miniscule and unlikely to have any effect on prices - at least, outside of limited areas.

I'm guessing at what you're referring to here so feel free to get specific. But cases in the USA are not that relevant to the UK, where the government is far more willing to enforce free market conditions than the "free market" idiotlogues in America who think that you can have a free market without regulation.

The Renters Rights Act overall benefits renters by making sure they can do such shocking things as keep a pet and make requests of the landlord without being evicted in revenge. This imposes a restriction on landlords, which amounts to a "cost" (in the sense of cost/benefit, not necessarily monetary) which means it was inevitable some would sell up. That part - landlords selling - does hurt the poorest, at least in the short term.

However, the effect will be small in the long run: because houses for sale will increase a bit in supply, those prices will come down, enabling more people to exit the rental market. That will bring down prices at the top of the rental market, and eventually a portion of the decrease in house prices will feed through to a mitigation (not a reversal) of the increase in rents at the bottom of the rental sector as well. So a small effect to begin with (because it's not like 50% of landlords will sell up without being replaced) and mitigated because while there's no perfect exchange between the rental and owner-occupied sectors, there is some exchange.

It's therefore a small price to pay for basic standards of decency for renters.

The true mitigation will come from building more social and affordable housing. The government has committed to do this but has so far abjectly failed, like many governments before them.