Something about the tax position of letting mortgaged property.

EDIT: I had misremembered. Landlords can still get a tax deduction for mortgage interest, but only at the 20% rate, not at their marginal tax rate. For most landlords with mortgages, this probably halves the tax credit.

In the UK, if a private landlord receives £1000 in rent, and pays £800 in mortgage interest, they pay tax on the whole £1000, not just on the money they made.

There are reasonable arguments on both sides:

A) Why should a landlord be able to deduct mortgage interest from their income, for tax purposes, when owner-occupiers cannot?

B) Why should a landlord be unable to deduct business expenses from revenue before calculating taxable profit, when all other businesses can?