As you will know, non-UK residents became liable for Capital Gains Tax on gains made on the disposal of UK residential property sold after 5 April 2015. If you exchanged contracts to sell the property by 5 April 2015, then you will have no CGT to pay in the UK as the gain will be exempt unless you return to the UK within 5 years of leaving if you left after 5 April 2013 or within 5 full tax years of leaving if you left by 5 April 2013.
Assuming that contracts were exchanged after 5 April 2015, you have a number of choices as to how to calculate the gain.
Given that you have never lived in the property and will not, therefore, qualify for main residence or letting relief, you will almost certainly be best off by choosing to use the 5 April 2015 value as your cost for CGT purposes, as opposed to the original purchase price. You can take account of the selling costs when calculating your gain. The first £11,100 of gains made by an individual in 2015/16 will be exempt from CGT.
There are two rates of Capital Gains Tax in the UK, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the level of your UK source income that is taxable in the UK.
A maximum of £31,785 of the net taxable gain will be charged to CGT at 18% with the balance if there is any being charged to CGT at 28%. If your UK taxable income exceeds your personal allowance of £10,600 for 2015/16, then the excess will reduce the amount of the gain taxable at 18%. In effect, the amount of the gain taxable at 18% will be less than £31,785.
Take a look here
for information on how gains made by non-UK residents can be calculated and the reporting requirements.. As I stated earlier, the default position would be to use the 5 April 2015 value as the cost for CGT purposes.
I hope this helps but let me know if you have any further questions.