The fantasy about the bunker bros is that they think that they will escape the doom.
They won't, of course. They may buy themselves a fortnight or so, but not much more. Their air purification system, or something critical, will go on the blink and the guy who knows how to fix it will be already dead.
Sure, but I still think that they will go down the drain with the rest of us.
England being an island, and having armies and a navy didn't deter the Vikings. And, in the scenario we are talking about, there will be Vikings.