It's unfortunatly another policy that massively favours the top earners.
For me - I'd get my 20% relief on my £1K mortgage. Grand. But actually, that doesn't quite cover the increase from £750 to £1K increase due to interest rate hike. It might take me out of paying almost any tax or NI - but what would happen in terms of contributions to my pension? Helpful, but has it's problems and I'm a slightly above average earner.
If you're someone earning £100K a year with a £3500 mortgage - well thats £42K a year in mortgage coming off the salary, plus pension. You're dropping out of the 40% tax bracket, you're getting 40% relief on the £42K compared to my 20% on £12K. Plus that's 42K of NI and 10% student loan relief too!
It favours those with higher expenditures, which are the highest earners.