I'd have to question the age a bit as most of that screams 1986, even the Tracer chainset. A lot of that stuff simply didnt exist until around 1983 onward.
Right!
That rear mech didnt appear until the 86 model year and those distinctive Lee Chi levers still scream 1986.
As the first MTB's in the UK didnt start appearing until 1983 (Ridgeback/ FW Evans/ Saracen/ Dawes) the demand for the All Terrain Bike wouldnt have kicked off until 1984 onward when everybody would have jumped in.
I bet you an XT rear mech of your choice that those bars are 21.1mm or '.833' as the Americanus Trumpus would have it - popular with BMX/ Schwinn and a few far east built brands.
Have a look in here
viewtopic.php?f=5&t=299323
viewtopic.php?f=1&t=295369&start=30&hilit=evans