Not sure when they moved to private practices but I suspect is was post 1989 which is when the pension changes kicked in and GPs started to retire early
They've always been private businesses. That's why the Doctors in a practice are called 'partners'. The own the business, and distribute what's left over after all expenses have been paid, between them.
I assume that things have changed but when I was installing networks into GP surgeries there were a number of ways of screwing a few extra perks out for them too.
The cost rent scheme. Practice get mortgage, buys land and builds new surgery. Leases surgery to local health authority at rent which covers mortgage and maintenance. Mortgage paid off (by local authority money) and surgery now belongs to doctors.
Post Graduate Continuing Education. Do a few courses throughout the year and get cash bonus for doing them. One Doc I know went on a PGCE course organised by one of the big pharma companies who, totally coincidentally, had a big push on to get GPs to prescribe their new drug. Course consisted of being flown out to Sun City resort in South Africa for 5 days, attending a 2 hour lecture in the morning and having the rest of the day free. All expenses paid of course.
Installing a new computer system. Pitch up to install a new system. At end of installation find one HP inkjet printer left over. This was not long after inkjets came onto the market and they were still very expensive. Ask the Doc where he wanted it installing - 'Oh put it in the back of my car, it's for the kids'. The system was paid for by the Practice who then claimed the cost back off their local health authority so his kids were getting several hundred pounds worth of printer courtesy of the taxpayer.