So for example - one of my houses. Bought 25 years ago for £60,000. Now worth say £400,000. I could be forced to sell. Supposing house prices remain stable (they won't). It would probably be 'valued at say £350,000 (the shadow chancellor already said that the valuation wouldn't necessarily be at market rate). On which I would have to give a discount of 110,000. So I would get £240,000. I would then pay 40 percent capital gains on £180,000. So in total I would end up with around £170,000 on a house worth £400,000. And that's assuming house prices remain stable. In reality there would be such a rush to sell (the most obvious thing to do for any LLs with less equity) that house prices would plummet. I'd be lucky to end up with £100,000.
Well, plus all the rent you've accepted over the years, which in the case of many BTL landlords is the method via which they pay off the mortgage. So, in that scenario, you would basically have made £100,000+ out of thin air just because you were in a position to get a BTL mortgage and the poor sap actually paying it off wasn't.
I'd be astonished if this actually became Labour policy, still less law, but as a person who's paid thousands in rent over the years, it sounds pretty fair to me.