The componsator just has a valve in it. When the rear suspension is compressed (dead body in the boot etc.), the anti-roll bar twists, which pulls on the end of the spring (if it was there) which puts increased tension on the compensator lever arm...
The extra tension on the lever opens the compensator valve more... and that allows more brake fluid to be pushed to the rear calipers.. so you get more braking force at the rear when the car is loaded at the back.. which makes it more stable.
With no spring, the rear calipers will always get just "minimum" fluid. This makes the brakes weakly, and it also stops the calipers from working very hard... so they are more likely to seize. (You never hear of front calipers siezing .. or hardly ever).
So, my guess is the compensator only alters the percentage of brake force from front to rear, not to each axle.. but it may be more clever than that inside...
But you can see the spring has to do some work.. so get the proper spring, not just some spring that's the right length.
It has to be a spring with attitude, to boss that compensator about.
Once you've done that... you might produce enough force in the rear brakes to pass the MOT.. and since the rear brakes will be working "harder" with the spring and adjusted compensator lever, that might help to keep them "free" and not siezed.
The brake discs at the rear should be completely clean. If you have 1cm of rusty mark round the edge, then that's the caliper... but see if they clean up once you have the compensator /spring sorted out.