> So the owner is only getting a 3.75% return on their £320k investment. That's not completely terrible, but it still essentially has negative value compared to the opportunity cost of selling the property and investing the proceeds. This is not a sustainable situation.

They will charge what they can. If they aren't happy with the return, they can sell the house and invest the money in DOGE or whatever, that means more houses for people to buy, higher supply leads to lower prices, which is good (with very limited exceptions - mainly of those in negative equity looking to move or remortgage)