The 6.1 rulebook clearly indicates that UK, Canada, FEC, Anzac, France, Poland, and Holland, only declare war on Germany when one of the above is attacked.
But… if none of those countries are attacked… there are no provisions for the allies to declare war on the European axis, and UK can only declare war on japan as of J3.
Lets propose propose Germany bypasses Holland, UK, France, and spends the rest of the game NOT at war with these powers. Germany can use her double impulse going around poland, and striking right at Russia turn 1 of the game! Then after the fall of Russia, it can choose to fight the built up Allies in Europe, OR, Build navy to fight the USA Independently!
Italy can help with both of these wars! Albeit, she has nothing to conquer other than Neutrals, like Turkey, Portugal etc.
I propose the political rules read that the UK and her allies can declare war on Germany if Germany has not declared war G1.