I have no clue how you do it in UK but can tell you how we do it in the US and how FAA looks at it because it is based on some common sense rules, nothing else.
Flying IFR in uncontrolled airspace is not specifically forbidden but you are on your own - no radar services, nothing. There is only one case when such flying makes any sense - you are sitting in some remote uncontrolled airport, got IFR clearance on the ground and somehow you must takeoff and climb (say in IMC) high enough to finally reach controlled airspace to be seen by the radar, this scenario can be specially amplified in mountainous terrain. Chance that someone else will be in this cylinder of airspace right above the airport is infinitesimal small (presumably you are on radio frequency for this airport) so this sort of flying does make sense. But I can't phantom any other scenario in which flying in IMC in uncontrolled airspace would make any sense.