The idea is good but the way it works isnt.
As the tennant you are resposable for the property. Paying for any repairs boiler services etc. The h/a doesn't put any money towards repairs no matter what % you buy.

But the biggest point is you don't get a house at the end of it. Even if you get up to 100%
You are buying into the lease which is the right to live there. The h/a can move you to another house. Sell the land from under it [freehold]. If you miss a few rent payments they can also take the house off you and not give you any of your money back

Shocking they can get away with it but it's all in their small print.