Hello and welcome to the site. Thank you for your question.
Your UK property income is chargeable to UK income tax and you should have reported it to HMRC whether you are resident in the UK or as non-resident landlord.
Having said that, as UK National you would be entitled to personal allowance as normal against your UK income and the profit would have been covered by the allowance and no tax payable.
You may be a penaltyfor late filing of tax return for the tax year 2011-12 and 2012-13.
You should register for self assessment now that you are back in the UK (if you already don't have a UTR number and report the rental income in tax year 2013-14 in Tax Return 2014.
If you have a UTR number then register at Government Gateway for self assessment and this would enable you to file a return online.
If the total of your salary and rental income in tax year 2014-15 is unlikely to exceed the personal allowance threshold of £10,000 then there won't be any tax payable.
More information on self assessment can be found herehttps://www.gov.uk/personal-tax/self-assessment
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.