Leaving to half-siblings

I am a British resident of France, have no children of my own and wish to make a will in favour of my three half-siblings, who are Britons living in London. Should I draw up a will under English or French law? 

There should be no problem in ensuring that your estate could be left to your chosen beneficiaries. 

Because of some international rules, a will valid in one jurisdiction is valid in another.

Both the UK and France have ratified the relevant convention, which means that either an English or a French will should work to ensure that your wishes are respected.

Given that you have no children there are no fixed inheritance rules arising in France that would require a set amount of your estate to pass to reserved beneficiaries.

Thus it does not matter if you allow French law to apply to the devolution of your estate, or whether you choose to elect for English law to apply in accordance with the EU Succession Regulation.

It will be important, however, to understand exactly how inheritance tax would be calculated; French inheritance tax increases the further the distance between deceased and beneficiary. See here.