Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
As you never returned to the UK and have decided to remain abroad, then you will not have exemption for the time you were abroad under the extended private residency rules, which allow up to four years as the proviso must be that you return back to the UK and either take up residence in this property OR move somewhere else in the UK due to geographical relocation of the same employer that sent you abroad. As this is not the case this is how the gain would be treated
UP until 05/04/2015 had you remained not resident abroad for a period of at least 5 full tax years, then the capital gain would not be liable in the UK due to non residency. And these new rules will over ride any normal review on the residency position and allowed private residence relief which would include the last 18 months of ownership.
However since 06/04/2015 the rules have changed - so you will only have a gain arising on the period 06/04/2015 to the date of sale, and I suggest that you arrange for 2 estate agents to provide a post April 2015 valuation
And then when you come to sell the property you must inform HMRC within 30 days - and I have added a link here that provides full instructions on how to proceed once the sale is final.
Let me know if I can assist further
Thanks for your further question
Usually there can are three calculations provides full capital gains disposal (if you are to be treated as resident and based on total ownership) and then a second if private residence relief is due and the third based on the gain since 06/04/2015 (you should be given information supporting each calculation as to what it relates to)
So much will depend on how accurate the information was that you supplied with the questions
You can use local estate agents to provide a post valuation (I would recommend at least 2)