Hi Peter

I searched in Google on 'compulsory rear seat bealts uk' and found this
quote

Seatbelts and the law

A decade of advertising preceded the introduction of a new law making
the wearing of front seatbelts compulsory in 1982. Soon after the law
came into effect there was 90 per cent wearing by car drivers and front
seat passengers. These high rates of wearing in the front seats of cars
have remained high. In 1991 when it became compulsory for adults to
wear seatbelts in the back of a car, there was an immediate increase
from 10 per cent to 40 per cent in observed rear seat belt wearing.

Belt up in the back

It became compulsory for adults to wear rear seatbelts in 1991, and
since 1994 we've focused on encouraging people to belt up in the back -
for everyone's sake.

Our previous seatbelt campaign centred on the very realistic 'Julie' TV
advert. It showed a car crash in which the unbelted rear seat passenger
is thrown forward into the driver - his mother - killing her.

www.thinkroadsafety.gov.uk/ campaigns/seatbelts/seatbelts.htm