From the April 6th 2015 if you have sold a property based in the UK you must report this sale to HMRC within 30 days of the sale. There can be penalties for non-compliance which are similar to Self-Assessment penalties.
HMRC’s infomation can be found by clicking this link.
If the property was your main home at one point when you were living in the UK, you can claim up to 18 months Principal Private Residence (PRR) Relief. This would be attributed to the last 18 months of owership.
You can elect to use the Market Value of the property as of 5th April 2015 as the base cost of the property. The gain would be calculated based on the sale value less any costs and the April 2015 value. There may be reliefs available such as PPR.
If you have sold a property in the UK whilst living abroad, please do not hesitate to contact us at firstname.lastname@example.org and include “Sale of property when I was non-resident” in the subject heading. We would be happy to assist you.