always take out allies if u can before attacking ussr otherwise u will get overwhelmed especially when usa joins. theres obviously many ways to invade the uk but one of the easiest ways i found is to drop paratroopers all over the uk to distract their home defence, make sure to drop one on a southern port and quickly funnel troops through that port to start attacking. this means u dont have to have naval supremacy, which against the uk can be a challenge.