There is an alternative to this, if the Anglo-Saxons land between the Schelde and the Seine. This is way less than optimal place, because France is a good place to defend from an offensive coming from the West. Lines after lines follow one after another suitable to take a stand.
The situation is similar if - as a second alternative - they would initiate the offensive from the Mediterranean (from Southern France or Spain, doesn't matter), the landscape also suitable for setting up subsequent defensive positions.
If a main attack would be led through the Balkans, that would need the military participation of Turkey, and a Soviet attack into the Danube basin, these could be stopped in the Rhodope, and the Carpathians. Same would happen with a main attack from Italy, just at the Alps.