The subject of Class C or D airspace was discussed a couple of years ago, albeit from a slightly different angle. GATCO was/is in discussion with SRG ref IFR flights at airfields with an ATZ only.
I agree with almost professional that the separation standards in Class D are sufficient. The workload for ATCOs and delays for VFR
flights would be unacceptable at many airfields in the UK if the airspace was changed from D to C.