I have never succeeded in getting a "popup clearance" in the UK, but the UK has an especially weird "watertight Class A" system.
The only other airspace (relevant to GA) in the UK is Class D and one can get transits of that OK, and VFR to IFR conversion is also OK there. But these are small bits of airspace. All the "enroute IFR" airspace is Class A and as I say it's hard to get into that.
Last time I tried it, they asked me for the entry point I wanted. I gave them one intersection and they refused that, saying that the base of Class A at that point was above where I then was. Obviously I could have joined CAS by a climb but they either wanted me to join it laterally (in which case where the hell was I going to find a usable intersection - there aren't any on the lateral edge of CAS are there??) or they wanted to be obstructive and wanted to make sure I give up. This was Manchester IIRC. I just gave up and say in some mucky cloud for the next hour...
If I want to fly IFR I just file a Eurocontrol flight plan for FL160, make sure I have enough oxygen, and get on with it
Europe isn't like America, evidently