I always understood colour deficiency affected c. 10% of males
https://www.nhs.uk/conditions/colour-vision-deficiency/
but moving on....
it may not be anything like the problem you think it is. Dr Mark Green of Motor Racing Medics has been working with MUK on this subject since way back last year, and has developed an LED lightbox test which MUK recognise as a valid replacement if one cannot pass the Ishiwara book tests.
I can only pass 3 of Ishiwara (not the 24 required) but the lightbox was not a problem. Once the colour test has been passed it is logged on the MUK system and you do not have to re-take it annually.
Try not to worry but I do recommend you give MSM a call. Don't forget you don't have to go to Gatwick as they run a clinic at Autosport in the New Year on a pre-booking basis.