What would be a number of missed plates which would trigger an UNSAFE reading?
(I did 16 with 2 missed.)
This is one of the great farces. The Ishihara test is considered passed with 2 or less plates identified incorrectly. As far as the manufacturer of this test is concerned you passed. How the CAA deem it acceptable to skew the pass criteria of an established test is beyond all of us, but they consider you have failed if you make a single mistake. If someone were to take them to court over it I have little doubt they would have a sore arse for a very long time.
Not particularly familiar with ATCO medicals, but I presume you'll need to head off down to Gatwick and do the lantern test again. You should only ever have to retake colour tests "on indication", for example if you've had a condition which might affect your colour vision (apparently there are some). Personally I would have refused to take the test, although that's not much use in hindsight.