On full surrender of the policy, you may receive a certificate from the policy provider, showing if there is a gain. This gain would be shown on SA106 in pouds sterling, in the tax year the gain arises. The official exchange rates can be found at:
Exchange rates from HMRC in CSV and XML format
To convert from a foreign currency to pounds sterling, please use the official rate of exchange at the time of the exchange. You are free to use the yearly rate, the monthly rate or the spot rate. If a conversion was undertaken to deposit the money into a UK bank account, then this figure can be used. If there is no gain, there is nothing to report.
HS321 Gains on foreign life insurance policies (2023)