There are specials rules that apply to UK property sales by non-residents. Since 6 April 2020 non-residents have needed to report and pay any non-resident Capital Gains Tax (CGT) due if they have sold or disposed of:

A CGT charge on the sale of UK residential property by non-UK residents was introduced in April 2015. Only the amount of the overall gain relating to the period after 5 April 2015 is chargeable to tax.

A UK non-resident that sells UK residential property needs to deliver a non-resident CGT (NRCGT) return and pay any CGT within 60 days of selling a relevant property. The return must be made whether or not there is any NRCGT to be paid. Even if there is a loss on the disposal and where the taxpayer is due to report the disposal on their self-assessment tax return.

There are penalties for failing to file the NRCGT return within the deadline as well as for failing to pay any tax due on time.

Source:HM Revenue & Customs | 10-06-2024