It shouldn't have an external fog light, it should be integrated into the normal rear light clusters.
Turn your rear fogs on and check that you get one of the rear light clusters illuminating with the fog lights. If you do you can disconnect the one under the bumper, as you need one for the MOT.
Then disconnect the battery, the light will be held on with a bracket, take that off, trace the wire back as it should go up to one the rear light clusters I would expect, and you should find a join. If it has been soldered on, snip it on the join wire and wrap that bit in insulation tape IMO, or if it has been spliced, remove the splice and again insulation tape to make sure you get no contacts off the +ve to earth.