Thank you for this most interesting question, and congratulations for planning ahead.
Let me answer you from the perspective of a EU-qualified attorney.
The facts are that you are a married couple, with one house in the UK and one (soon) in France, and two children.
Regulation 650/2012, about which I wrote a book, does apply to you for the French property, but not for the UK property, because the UK never opted for it. This results in a situation where each of you has two estates, one containing the UK house, and all your movable property, and the other one containing your French house. Consider it as two buckets, not one, as a person normally would.
The UK bucket is governed by the law of your domicile, i.e. of that part of the UK where you are domiciled (as you know, there are in the UK several systems of law, the most important of which are English (including Wales), Scots law and Northern Irish law). Your Will, being under English law, would be valid there.
The French bucket is governed by the EU Regulation like this: it says that the law of the "last habitual residence" of the deceased applies. In your case, that would be UK / English law. However, this law "sends back" (the rule is called "renvoi", even in English) the issue to French law, because that is where the house is situated; and French law and the Regulation accept this renvoi. In other words, finally, French law applies to the French bucket.
Your English will would be valid according to form in France, because under the Regulation, a Will is valid according to form if it is valid under the laws of the place where it was made, i.e., the UK.
Now, regarding the IHT, I would say that the arrangements you describe would fall under IHT. See in particular this page (https://www.gov.uk/hmrc-internal-manuals/inheritance-tax-manual/ihtm27000), items 27211 through 27230. However, this part of the question would be for a UK tax adviser, and not an attorney.
I hope that my answer was helpful and look forward to you rating, which is essential to my activity.
Dr I L Vlad