I am sure somebody will correct me on this, but AIUI there are about 4 different CV tests approved under JAA, and once you have done one of these (and failed it) you are not allowed to repeat that test ever in your entire life. You have to move to one of the others. So pilots shop around.
Many people fail the initial test - the Isihara plates. This test is a meaningless indicator of CV anyway, as a recent study commissioned by the CAA (I read it but wish I could find the URL again) confirmed. So they have to do one of the others.
The advantage of doing the CV test in some far away place is that a method may be available which is not available in the UK. Or that a failure is not recorded so you could later try the same test again?
The problem is on the JAA initial medical. On a renewal medical you are not tested for CV, and you can invoke demonstrated ability on a lot of things anyway. The JAA initial/renewal system is a farce.