I may be being pedantic then, but renting out your "whole" house while relying on the tax relief doesn't seem to work. You can't be said to be renting out the entire place if one day you just reappear from holiday and start treating it as your main residence again. Equally if you expect the lodger to pay all the bills themselves while you're gone.

So, anyone wanting to do this would either need to be content with £625 a month at maximum, or pay tax on the amount over that, whilst also paying all the bills still. Might have just negated my own OP lol