You can't decide how your French assets will be disposed of (at least not very much). If you have children it will go to them in set percentages even if you are domiciled in the UK. I think that for clarity sake you add a clause to your English will which says what you'd like to happen to the French part of your estate, but if this is at odds to French law (which is complex and inflexible especially with non movable assets (house and land) it will be overruled. The law is basically biased towards every child getting something and the estate therefore being split or sold off. It's all pretty bonkers stuff and non-negotiable as I understand it.
Take some advise on a French will. Leaving anything to non-close family member can be expensive with 40% or more payable from the first Euro in some cases in death duty (IHT). If you have something you wish to leave to a family friend make sure it is part of your UK estate if at all possible, although if you are domiciled in France that won't help so much.
The rules were changed recently and are much better, but still nothing like the UK.
There is a nice overview here.