I agree with Farangatang.
You will be taxed on the net income, and you should decide whether you get your agency to deduct tax at source or register as non resident landlords. You'll have to register anyway, but we get our agents to deduct tax anyway so we don't have to worry about that from month to month.
So...and I hope I've got this right...you'll have
Gross income... then deduct
Estate agent fees
Mortgage interest - not on interest only mortgage though.
Repear jobs amount
Insurance
You'll be taxed on the amount there after...you can also claim 10%for wear and tear
So it's 20% of what's left over.
If your agents do the tax, then you'll need to refer to their notes too.
You'll also need to keep a note of how many days you have been in the country, i.e. UK for the year you will be filing a return for. This will go in your self assessment.
Also don't forget that if the mortgage is in joint names, i.e., you and your OH, you will both need to file a return. HMRC see it as a 50/50 split regardless of who pays the mortgage or receives the income from any rent.
That's a joint mortgage.
I am no expert, but that's how I understand it.
Check the gov.uk website, it will have all you need to know.
If any other poster has extra info, I'd be keen to hear about that too!