Thanks for asking the question.
Firstly this is a complex area - an "Assurance Vie"/ Life Assurance is a French version of the insurance bond. There are no French income or capital gains tax on the gains/income accumulated within the policy if one doesnt make any withdrawals. Life assurance bond as it is called in UK, has a number of tax implications whether is the act of a withdrawal of funds, whether as a partial surrender or a full encashment of the contract. There are also tax implications on the gain of the value of the insurance irrespective of whether funds are withdrawn or not.
Unfortunately the double tax treaty between UK and France does not mention Assurance Vie/ insurance bonds making things slightly complex. In summary the Double tax treaty states that any credit given for taxes paid in the UK is given in accordance with the manner in which France would tax the income/gain - BUT given that France does not tax such gains/income at source on these contracts, there is no credit calculation for whatever tax you pay in UK.
Therefore when you make a withdrawal you are charged income tax at nil if the personal allowance is available; starting rate 10%; basic rate 20%, higher rate 40% and additional rate 45%. Top slicing relief is available for the higher rate and additional rate calculations. However the exemptions and reliefs; and capital gains tax calculation is not as simple as stated above. In most cases, your insurer may supply information about chargeable events to policyholders and HMRC directly or through a tax representative to work out the tax impact. In practice, one would get advice from a tax specialist or IFA from the insurance provider as this a complex area.
Finally as this a complex matter and i would advice getting a specialist tax advisor to deal with your query more specifically. Ideally get a tax expert with both French and UK tax in life assurance bonds. If you cant find one online googling try this site HERE
where you post your specific request with as much detail as possible and a tax adviser near your area contacts you.
Hope this helps