This is the BRNAV equipment requirement for which an IFR GPS installation is the only possible "GA" means of compliance.
Currently BRNAV is mandatory in CAS for IFR above FL095.
They are proposing to lower the FL095 figure. I am sure this will apply only to IFR flights on Eurocontrol flight plans (often called "airways" flights in the UK) which need the full IR anyway (the IMCR is practically useless in this context due to most UK CAS being Class A, and the IFR privileges of the IMCR are not valid outside the UK).
It won't apply to VFR flights in CAS - it cannot because "VFR is VFR" and one cannot mandate BRNAV or any form of radio nav on VFR traffic which has ostensibly been taught nothing beyond "WW1 map+stopwatch" flying. In the same vein ATC cannot require IFR traffic to report at a VRP
I am also pretty sure it won't apply to IFR flights in Class D e.g. an IFR transit of the Solent Class D via SAM at 4000ft, which is routinely done by IMCR pilots. If BRNAV was mandated for these transits, it would make a mockery of the IMCR privileges and equipment carriage requirements because the IMCR would be constrained to Class G, forcing a lot of IFR routes below the MSA and closing off instrument approaches to many airports.
If GASCO says what you say above, it would not be the first time they printed garbage.