Well.. thats where it might fall into abit of a grey area.
I would imagine the mounting points for the oem seat belts would suffice as far as the Mot was concerned.. however, in terms of "harnesses/motorsport/rollcages" etc etc the harnesses should be mounted at roughly a 90 degree angle to your shoulders, or atleast angled at 90 degrees with the aid of a harness bar.
With the harnesses mounted into the rear seat belt positions they are angled incorrectly and can actually do more damage than good in an accident by pulling your shoulders downwards.
With that being said though most road cars with harnesses seem to mount them this way anyway (but that doesnt make it right).
I have harnesses in mine and have had a harnesses bar welded into my rollcage.
The correct way to use a harness bar is like this:
The harness is still mounted to the floor but in the event of an accident the downwards strain on your shoulders is deflected away by the bar.
Some people mount there harneses directly to the bar.. this is also wrong unless otherwise stated by the manufacturer of the harness bar.
I did alot of research on all of this while fitting my harnesses.
Perhaps dont take everything i have said as gospel but just as a guide, i am only reciting what i have heard and read from other sources.