Well I would disagree, Mr Radar.
I know of someone who has successfully completed a return trip to the south of France from Scotland "sans transpondeur" - VFR all the way, but helpful controllers in CTRs and a bit of prior notice and a lunch emergency .... and it was perfectly feasible.
No reason why you can't have VFR routes in Class D airspace - such as the entry/exit lanes at Glasgow. Or in SVFR routes in Jersey/Guernsey CTAs.
Mandatory is overkill.
But that's exactly what I put forward
Mandatory carraige in Controlled Airspace or above FL100 but with agreed exemptions, be that through the use of Letters of Agreement and RA(T)s, published VFR routes and conditions attached, or Prior Permission Required from the ATC unit involved if not either of those. Some of that will help gliders, some won't.
I am not sure that the Emergency Lunch exemption will catch on in the UK. The food at UK airfields is too bad
