Would you be OK with everyone's mortgage payments going up by 20%? Think about what that would feel like for you. It's a similar amount.
Even if you could manage it, you can see that would cause serious hardship for some people - who may have overstretched themselves, or may just have had a change in circumstances. They were just hanging on, because selling their house would be a huge hit, but now they just can't. Or else just don't want to: renting is now a better choice for them.
Now imagine that the government did this deliberately with a 20% Mortgage tax, purely because they think that house-ownership is right-wing and immoral.
But despite it being a tax, no money would actually be raised, because the government had simultaneously created a scheme where if people defaulted on mortgages, they would pay the bank (not the householder) the whole cost of the house (The householder still loses their house.) They didn't bother doing the calculations, but other people did, and it seems likely that so many people will default - which the government will pay out to the banks - that it will lose the government money.
So all that pain for people, and no money actually raised.
And loads of people on mumsnet were cheering it on, and saying that they hoped those stuck-up house-owners ended up penniless on the street (like they've said they hope that stuck up rich kids get bullied in state schools).
Maybe you can afford your newly increased mortgage + 20% we-hate-you-tax, by tightening your belt a bit. Maybe you can't. But you can surely see that you'd be fairly pissed off.