Letting out existing property to buy new home

Funny_old_game

Glad to say this sort of behaviour will be less from tomorrow.

Typhoon2000

Remember you will be entitled to tax relief on the value of the property you introduce into the letting bussiness. So depending on the size of the mortgage you take on the new residential property, you may be entitled to £600000 of interst tax relief, not just the amount you raise on the BTl mortgage. So if you borrow £300000 on the BtL you could still still effectively have 100% tax relief on the BTL interest even after the tax changes are phased in.
Plenty of people will need/prefer to rent. If the supply of rental properties goes down, expect rents to go up.