If your sensors all test okay (ohms should be around 900-1100ohms) and the ABS sensor rings are fine, then try checking the wheel bearings.

Sounds silly, I know, but I had this problem on my 4be, but the light would come on randomly and then stay on until the ignition was turned off.

The car passed an MOT fine, all sensors were checked and exchanged for other known good ones and still the problem persisted. Decided to check the wheel bearings, just in case, and both passenger side ones had about 1/4 turn before going solid. This was using a 3/4" breaker bar and standing on them. The bearings go to something like 330lb ft on the front and about 250lb ft on the rear. Mine were still tight enough to hold the bearing, but when under load, allowed the ABS sensor ring to move just enough to put the air gap out of spec with the ABS sensor.

Might not be the problem, but might be worth checking before you go looking for something that might not be there.