Rent Controls abroad (well if everyone else is throwing their hats into the ring)

The Swiss have a civil code legal system rather than a common law legal system (unsurprisingly). Broadly what this means is they look up their gigantic book of obligations or other area of law and apply what that says. In England and Wales you look up what Judges have said about the vague witterings of parliament and in Scotland you look up what Judges decided to do on that particular day and try to figure out if your judge will be subject to the same whim. 

As regards unfair Rents, Switzerland tacks this onto the end of the general obligations in relation to leases. I am not sure why just transplanting something like this to the UK would not work.