The European Union can collectively defend itself against any conceivable invader. What it can't do without US help is bombing countries far away, as it lacks the military logistics and force generation to do that well. That's the main purpose of NATO since 1991, and the UK along with France really like having the option to bomb meddlesome African countries into civil war, so they play along.
There is no scenario in which Russia could successfully invade an appreciable part of the EU, even without taking into account European nukes.