the fact they're private businesses only contracted to the NHS is a clue
I may be wrong, but I seem to recall reading somewhere that there was considerable resistance within the medical establishment to the setup of the NHS and the continuing ability of GPs to operate as private businesses was one of the conditions imposed on Atlee's government.
Tim H. 1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE