If you were speaking of the UK and France as their own nations - as we think of them, then... Yeah, Sure.
When speaking of them as captured territory, then, why would the captors care about what happens to them?
Ok.
So, how many kilotons can they actually fire at the US and how many will actually make landfall before both countries get turned into radioactive nuclear wastelands?
Together they can deliver about 65 megatons - if they're really lucky.
The US can deliver almost 1000 megatons.
And it's about 10 000 miles away - which makes anything fired at it easily detected and allows plenty of time for a counter.
So, tell me again about France and the UK's mighty nukes?