Thank you all for your input.
I spoke to Motability today and told them that I had purchased a private registration for my mother and told them the reasons for doing so, they accepted and said I could add it to our current vehicle, but I told them we would add it to the next one instead which they said was fine and would make a note on the system. They did say just to make sure with the dealership when getting next vehicle that they can process the registration or I will need to contact Motability 7 days after taking delivery for them to do it.
Humanity is only humanity when all are treated the same regardless of their skin tone, religious beliefs and geography!