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.
Just discussing this in the office, with Bevan's pledge to 'fill their mouths with gold' to encourage them to accept the NHS alongside their own private patients.
"Y mae dafad ddu ym mhob praidd" 1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10