Techncally they are illegal, but I have had a light bar on my bike since 2004 and never had any trouble with the cops or VOSA.
With regard to the MOT, just turn them off at the switch on the LH lamp - they are not a listed test item so as long as the are not on when they do the light test, they tend to ignore them.
Just make sure that they are set so that they don't dazzle anyone and I am sure there will be not problem.(but don't hold me to that!)
