Assets abroad can complicate things HUGELY.
If you want to do things properly, you MUST get proper professional advice from someone who is an expert in both jurisdictions, which can be expensive.
The UK is one of the few EU countries which has opted out of EU rules on inheritance and succession
https://europa.eu/youreurope/citizens/f ... dex_en.htm
And this is now, before Brexit.
Also, the UK and the US did not ratify the Washington convention on international wills.
Another thing to bear in mind is that the applicable law might change if your residence changes. Eg you have a house in Spain and you might move there when you retire - should all your assets be regulated by Spanish law?
I don't know the answer to that, but there is no substitute for proper (and potentially expensive) legal advice.
PS Many civil law countries have rules on the minimum amount which wives and children must receive; giving them less is almost impossible, and allowed only in extreme cases (eg heirs who had tried to murder the deceased). Interestingly, this has nothing to do with fairness, but is something which was introduced by Napoleon as a way to reduce the wealth, and therefore the power and the influence, of the wealthiest families.