You have three choices really.
1 You can base your CGT liability on the value of the property as at 5 April 2015. What that means is that the cost of the property for CGT purposes will be whatever it was worth on 5 April 2015 (£350,000?) and you will only be liable for CGT if you sell it for more than that. In fact, you will be entitled to an exemption from CGT for the first £11,100 of the gain assuming you sell it in the current tax year. See section 6 and paragraph 7.7 here
As the property was at some point your main home, you will be entitled to exemption from CGT for the last 18 months of ownership and to letting relief if the property has been let during your absence.
The above should pretty much guarantee that you won't have any CGT to pay in the UK.
2 You can base your CGT liability on the entire ownership history and original cost which would be the case if you lived in the UK. You will be entitled to main residence relief for the period that you lived in the property and for the last 18 months of ownership and to letting relief if the property has been let in your absence. You may also be entitled to absence relief instead of letting relief for the entire period that you have been abroad if you left the UK to work but only if you re-occupy the property before you sell it. See paragraph 7.9 here
3 You can choose to base your CGT liability on the whole period of ownership but use the original cost and split the gain between the pre-6 April 2015 and post 5 April 2015 periods. See paragraph 7.8 here
I hope this helps but let me know if you have any further questions.