I've always had problems filing /Y or /Z. When flying VFR first to join controlled airspace, London have failed to find my plan in the system every time and when wishing to leave controlled airspace (shortcut from DVR to Fairoaks via BIG without having to route via the whole south coast IFR), London have been quite short and dumped me out of controlled airspace over the Kent coast rather than handle me IFR toward Biggin then a radar info service from Farnborough (which is what I wanted).
Last Sunday I filed IFR from Plymouth to Fairoaks, mainly to stay on top in 100km viz rather than 2400 feet in bumpy CU and <10km below the TMA. Without radar, Plymouth suggested a VFR departure to "pick up a joining clearance from Exeter". Despite having a print out of my "OK" flight plan, Exeter, London, Solent nor Bournemouth could find me in the system so approaching SAM, I gave up and stayed clear of CAS.
The system works with controlled airspace and/or SIDs/STARs at both ends but trying to cobble together an IFR clearance without it seems to be a big hassle.
I've got the rating and Class A airspace is safer. My tax money pays for it, my AUW <2000kgs means there is no extra cost, so I'd rather use it. My experience of making it work is somewhat different.