As someone who was on the receiving end of ATSOCAS many times during my early regional flying days, I can now unequivocally state, that the current array of services is the most complex state variance with ATC procedures that I have seen anywhere else in the world. The standard of controlling throughout the UK has always been meritorious in nature but service provision outside of CAS, is a confusing affair for many with no, or limited exposure to ATSOCAS in the UK.
Whilst responsibility for the pilot being aware of the different levels of service provision with ATSOCAS, is irrefutable, the practicalities of always achieving that may be challenging. Take the Air Ambulance pilot who is called out at short notice standby to fly from abroad and into a UK airfield in Class G. There needs to be an advance awareness that part of this flight will take place outside of CAS in the first place. Secondly, that pilot would need to be aware of the different levels of service that exist in ATSOCAS. Thirdly, they would need to have the time and the information to hand to ensure that there was familiarity with ATSOCAS in the UK, prior to being asked 'What service do you require'?
I now rarely need to use ATSOCAS but on a couple of occasions in the last few years that I have had to, my co-pilots on both occasions (who were not from the UK but still very experienced) were at a loss as to the different levels of service available in ATSOCAS. One of the exciting things about aviation is that the pace of change in technology advancement, has transformed how we do our job but it would seem that there is perhaps room for some transformation with how ATC is implemented in current Class G airspace in the UK.