Has the car already failed the MoT because the sensor is defective? I wasn't aware that this was a failure point as ABS not working doesn't affect the brake tests carried out.
I tend to disconnect any ABS systems I've had on previous cars because they can make stopping distances even longer! My cars have never failed the test because I've disconnected the ABS.
If the tester is complaining about the warning light, just take out the bulb and go to a different testing station!!