Its down to the tester more than anything else. Best bet is to ask them before you go down there if they'll fail you if you don't have it
My 916 has an LED bulb in the standard holder (normal bulbs break due to the vibrations) and because it is a little longer than the original bulb it is mostly inside the casing so the plate illumination is quite dim. During the MOT test a couple of years ago I asked the tester about it and he told me that it didn't matter as a number plate light is not part of the MOT test. I'm sure it's a legal requirement but, like a working speedo, not required for MOT. Unless the rules have changed since?
