I was thinking of altimeter settings changing on route. I suppose the regional settings would do most of the time. As SK says it works fine in the USA.
Discussed in a fair amount of detail in the CONOPS discussion
here.
However am I right thinking there is no IFR outside controlled airspace in the rest of Europe?
No, SERA permits it, and in fact some of the "rest of Europe" in which IFR is not currently permitted in class G is [hum=DambustersTheme]having to have a rethink[/hum].