Where would they want to go, if zombies are everywhere?
Also, keep in mind, that land on large landmasses only seems so easy to cross, because we use cars, bridges, etc. Zombies probably wont do that, so things might not be all that bad.
Yeah, the biggest problem for British Isles would be hunger, as even during WW2 hitler nearly managed to starve them, and now they have even more people.
India and China? keep in mind, that while their populations are huge, so are their areas. I think china has population density similar to poland, and its not that much. India probably is similar. Think about Bangladesh, and other small and dense populations like that. Benelux, comes to mind, also. (also, no guns there)
Anyway, the zombies dangerousness dependst mostly on how they work- if they decompose just as dead people, zombie invasion wouldnt be a problem, especially in equatorial area. Also, during winter in sub zero temperatures, we would be safe.
And, as usual, i agree with Dodom:P