Is a will and power of attorney made in France valid in the UK?

There may be omissions due to different legal requirements.  E.g., some terms required in the UK, or in France, might or might not be in the Will.    Or French mandates might be included that the UK wouldn't necessarily honor, and that could cause difficulty in administration.

Either of these possibilities  could present a problem after death.

I'd check with an attorney in the UK to ensure that the will is compliant with UK terms.

Wills are generally recognized across international boundaries. But if there is a testamentary Trust written into the Will, you could be faced with jurisdiction issues.

If a person normally resides in a country and dies outside of it, then the Will will stand.

If a person has permanently moved to another country, then it makes sense if they are competent to update the Will. The rule of Thumb I’d to update every time there is a major life change. Buying or selling a home, moving, births or deaths etc.

POA laws are more regional.

Check with an attorney, or solicitor as they are called on the other side of the pond.