On MIRAS, yes, I don't think I said otherwise - it was done via PAYE (as part of pension payments are in a sense today - private pensions are paid after deduction of tax).
MIRAS etc is one reason taxes today are just as bad as in the 70s albeit there were higher rates then because my father had things like child benefit, single person allowance, MIRAS, could covenant money to me at university and have tax relief on that. All that went and although upper rate of tax is now 45% (and 2% NI on top and 9% student loan etc on top of that) thigns ilke university child benefit disappeared as did no student fees etc. So we now have the highest tax burden in 70 years.
My link above about the position before MIRAS and during it sets out some of that system (and in the USA I think you can set your mortgage interest against tax). MIRAS was gradually reduced over time so I never really had much benefit from it.
The Guardian has a bit of a summary of it from 1999 article here
"Miras has been eroded over the last few years. Tax relief is now restricted to 10 per cent on mortgage interest payments for the first £30,000 of a loan. It is currently worth £17.37 a month for the typical borrower with a mortgage over £30,000 - up to £208 a year.
Tax relief was originally allowed on interest paid on any loan. In 1974 it was restricted to mortgage loans and limited to £25,000, and the scheme as we know it now was introduced in 1983."
https://www.theguardian.com/uk/1999/mar/10/budget1999.budget3
So that suggests the tax relief by 1999 was only 10% on the first 30k of a loan. (In 1999 we had a mortgage of about £500k). I presume that means limited to a tax set off of 10% so if you were a 40% tax payer for example you were not by then getting the 40% set off.