Most english fans support the other home nations when England aren't playing so it's the sensible, grown-up thing to do really.
It's not sensible, or grown up. It has nothing to do with either of those characteristics. The fact it'd bother you someone is apathetic or rooting for another nation in the WC is childish to me.
I'll pull for England, but I admit I love watching the English and how they react to their team. It is really great. An Evertonian stops into where I work and we were talking, he's from Liverpool. So I ask him, do you have a side you like... With so much resentment and disgust in his voice he immediately quips... "Certainly not England". The media and fan scrutiny is amazing. Maybe it's because they're the only English speaking nation, with the combination of being football mad, capable enough, and developed enough to have a real media. I'm sure German, Brazilian, and Argentinian media would be equally as entertaining. I'm just inept at those languages so it's not as entertaining.