Hi Malise Pang,
Tax in the UK is calculated on your world-wide income on the 'arising basis'.
This basically meant that something is taxable in the tax year in which is arises.
You would only show the overseas income and gains in a tax return, in the tax year, when the policy matures or is redeemed.
Further guidance can be found here:
Gains on foreign life insurance policies (Self Assessment helpsheet HS321)
Thank you.