In the UK, flying under IFR out of controlled airspace is a captain's decision, you don't have to tell anyone, you just decide you do, and provided you're above MSA and fly according to the quadrantal rule (rules 33 and 34), and have at least an IMC rating, it's legal. If the weather is fine, it's just like flying VFR, but you may legally log IFR time.
If you have an IMC rating, and fly above MSA according to the quadrantal rule don't forget to log IFR time even if the weather is fine
In some other countries, for instance France, you may not fly under IFR, even out of controlled airspace, without a flight plan.
Last edited by 172510; 16th Mar 2013 at 16:41.