The UK answer is - it depends on the airspace category. Having said that, if you're talking about class G airspace which is virtually all of it outside control zones and airways, then you don't need to file a flight plan, get a clearance or even talk to anyone. It is probably sensible to talk to a radar service, but in the South East you sometimes get wonderful replies to your request like "unable to give you radar service due to high traffic density"!!
For ops inside class A airspace that are not following established routes you need a special flight clearance. Call the relevant ATC unit and they'll tell you what you need to do to apply, and give you advice on whether it's likely to be approved!
Good luck.