Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
I am afraid you cannot designate one of these properties as your main residence as you are not physically in the Uk - however there may be relief due, so please advsie why you left the UK - for example was it due to a post with an existing Uk employer - and how long do you plan to remin abroad, and how many visits have you made back to the UK (and for how long) since 2015
Are you declaring rental income to HMRC?
The gain is always calculated with the sale price less acquisition price to form the intial gain - from which releifs are considered to mitigate the gain arising, hence the questions above
Thanks for for your response
You advsie you lived in house 2 FULL TIME until 2008 - what happened between 2008 and 2015 when you emigated - and have you moved abroad permanently.
Have you declared rental income to HMRC?
Thanks for your response
No you should be registered now with HMRC as you are situated abroad and unless you have agreement to operate under the NON resident Landlord scheme then tax should be deducted by the managing agent and paid over to HMRC quarterly. So you should have registered as soon as the rental income began. Please rectify this as a matter of urgency. Please refer to your managing agents who should know this!
However back to the Capital Gain position - as you seem to have been treated as Not Resident since 2008 - then your capital gain will arise just from any gain made from 06/04/2015 to the date of sale (due to changes of legislation) link here re those changes https://www.gov.uk/guidance/capital-gains-tax-for-non-residents-uk-residential-property
So make sure you get a valuation for the original property as at 6th April 2015.
So the longer you hold onto the proeprty the greater the gain to consider - but as UK citizens you will still be entitled to annual exemption allowance (currently £11,000)
Let me know if I can assist further
No get a solid valuation - estate agents are able to provide post valuation (in this instance get a couple) as are the district valuers office (whose valaution would be solid) - see link here
Do let me know if I can assist - and do get onto the managing agents - as you say, one would hope they are following correct procedures and deducting basic rate tax from your rents to pay over to HMRC OR have applied for the Non Resident Landlord scheme
Do let me know if I can assist further - however it would be appreciated if you could take the time out to rate me for the level of service I have provided you with (or click accept)