I had this on my last MOT. The plastic lens had a stone crack. The Examiner said he could not fail it for that as it fulfilled his schedule as follows
Stop lights, indicators and hazard lights
are the correct type and colour
do not interfere with each other in operation
driver's tell-tale works with respect to indicators, or there is an audible warning system.
I used to have a car where a stone had hit the headlight. A piece of glass had broken off the inside and there was a very small hole through to the outside.
I repaired it with a small piece of heavy duty transparent tape, and it passed several MOTs with this.
any lens broken or cracked is only an mot failure if it results in a sharp projection or if it is insecure to the extent it would fall off, also it fails if it results in the colour emitted differing from the one it is supposed to . ie if it was a red or orange lens and the crack causes it to then emitt white.
like panic button one of our lights has been cracked for a while (husband- car-lamp post!) heavy duty tape has got it through several tests. However this year no tape would hold bits of the chassis together!! :-)