Probably the best option here would be to buy the car and insure it as normal on a 12-month policy in the UK. The majority of standard insurance policies allow up to 30 days European cover despite Brexit so providing this is in your policy you can then drive it to your holiday home in France. Once it is there, you can then arrange to re-register the car in France and take out a domestic insurance policy there, remembering to complete all the correct export documentation with DVLA here. Once the car is in France and you are home, you can cancel your insurance policy but you may need to pay a small early termination penalty.