To the best of my knowledge, it's also a requirement for aircraft entering the UK FIR (as opposed to controlled airspace) from an adjacent European FIR, and not working an ATC unit.
I think that the converse may apply to entering one country's FIR from another within Europe under the same conditions, but I am not certain of that.
It seems to be an universal ICAO procedure and you'll probably find it stuffed away in one of their enlightening documents somewhere.