Made up by the pilots usually with no sanction by the company.
There is no EU rule stopping you flying IFR without a plan.
Over 7500kg you have to have a flight plan anyway be it VFR or IFR.
Some country's though may get ****ty with you or have differences files. As such ATC services are not under a common EASA set of rules yet and there are quite large differences between country's.
And UK has the most differences I might add away from ICAO.