Sorry, you`re quite right, there are some exceptions as well:
If a vehicle has no seat belts (eg a classic car), a child aged
3 years and over may travel in a rear seat unrestrained.
A child under 3 years old may not travel in cars, vans or goods vehicles which do not have seat belts installed.
if there are no seat belts fitted in the vehicle, then a child under 3 years old may not travel in it.
My vehicle is a camper van with no rear seat belts
A child under 3 years old must use the correct baby/child seat, such a child could only travel in the van if the child restraint can be used with a front seat belt. A child of 3 years and above and up to 135 cms in height must use child seats/boosters in the rear where seat belts are fitted but may travel unrestrained in a rear seat. The new rules do not require people to fit seat belts in the rear of vehicles where the seats do not have seat belts, but remember it is not safe for children to travel unrestrained. Vehicles dating from April 1982 have seat belt anchorage points if they don't have seat belts - so fitting is easy.
Can`t find the date of manufacture after which rear belts must be fitted, had a trawl round the Gov website but it`s not obvious.
I would have thought though that it`s best to make sure you have rear belts fitted anyway, they are after all there for good reason

An unrestrained person/child flying forward is likely to knock your head clean off just before they exit the windscreen
E D I T: dates for seat belt fitment, fronts was from 1983, rears was from 1991.
Keep it real.
Search first - ask second ;>}