Found this:
Are emissions on hybrid vehicles an issue? The simple answer is yes. There’s a lot of confusion when it comes to hybrids and mild hybrids. The majority of drivers don’t fully understand the meaning of each and how it actually works. Currently, all hybrid vehicles don’t require emissions testing...
ccmhelp.co.uk
"Currently, all hybrid vehicles don’t require emissions testing during their annual MOT however, we don’t think this is right... We have created a petition to get the government to look into this."
Sounds sensible, but I am just thinking that if the car is presented to the test with a full HV battery, how can the tester initiate the ICE engine startup to check emissions? It would be very difficult to carry out MOT tests on vehicles where the engine only runs when the computer says so. There may be an option to force the engine to start, but even so it will vary between marques.