Flying Blind.
I looked on the CAA website for guidance and this is what it says
(b) Examination
(1) Applicants shall pass the Ishihara test for the initial issue of a medical certificate.
(2) Applicants who fail to pass in the Ishihara test shall undergo further colour perception testing to establish whether they are colour safe.
My bold. It would seem from that that they consider the Isihara to be the main test BUT also accept that it is not the ONLY test. Give the Belgrano(CAA Gatwick) a ring and discuss it . Very best of luck
EW