I pay roughly 2% of the equity I have invested in tax on rent received less expenses annually, so getting rid of all tax would represent a 2% bump in yield for me.
They are never going to do that, but at that level I'd have a decision to make, holding on for 15 years that extra yield might compensate for a 30% loss in value on selling due to e.g. a market crash or selling with tenants in situe, or it would cover lost income for the lengthy eviction of a non-paying tenant.
If they matched the rent a room relief of first 14k as others have proposed on here, presumably you wouldn't be able to deduct any expenses on the excess, that would be taxed at 52%, so it wouldn't actually work out as a huge saving for me. I'd get an improvement of roughly 1% in yield terms, it wouldn't cover the cost of the risk events above. Personally it wouldn't keep me in, but in conjunction with other measures in terms of control of your property and avoiding regulations that impact on sale value it might.
Reducing the overall tax level to 20% would be similar in that the tax savings would be (very roughly) 1% bump in yield.
Edit:
for rental owners with no mortgage the impact would be very different of course as they have no mortgage interest to offset profit.