We live in the US and often travel to France.
It has actually prevented us on multiple occasions from spending an extra 2-3 day family holiday in London combined with an EU trip. It won't kill us, but why spend an extra $500-700 if we don't have to? The per diem amount isn't palatable.
What?!
If you fly US to UK, break your trip then continue UK to France, you will only pay 4 x short haul economy APD.
That is $100 max for 4.
Breaking your trip on the return will I admit be $700.