Hi Susan Wong,
UK resident customers are chargeable to CGT in respect of any gains arising in the year of assessment in which the disposal is made, this includes foreign disposals. If a chargeable gain accrues to an individual in a year when they are not resident in the UK or the overseas part of a split year then normally they will not be chargeable on that gain in the UK.
If the CG chargeable in the UK is also charged in Hong Kong they should be able to make a claim to foreign tax credit under the DT agreement in place.
If customer is UK Resident, and not claiming remittance basis, they need to declare the full gain arising on the sale of the property in Hong Kong, based on UK CGT computation of liability due. I believe that they are interpreting TCGA92 differently to HMRC. They do not apportion the gain between Non-UK residency and UK. If the disposal arises/occurs in a period in which they are UK resident and there is a gain arising the full gain needs to be declared in UK. Only exception to this is if they are classed as temporary resident, or claiming to be non-domiciled and claiming under the remittance basis. I believe HMRC's choice of wording ""accrues"" is some what misleading.
Tax when you sell property: Selling overseas property