Dr Smurf,
First good luck with the tests at Gatwick. They will first do the ishihara test under a lamp. You are allowed 0 failures and must answer without hesitation. If you fail that then you will do the Baynes lantern test. This is a lantern where you sit 5 metres away. It has 5 colours, Red, Green, White, Blue and Orange. The lights are about 3mm in diameter and are only shown for one second you must name them all without error. If you fail that then you will do the Holmes Wright Lantern. This you sit 6 metres away and there are two very small lights in it (about 1mm diameter) the colours are Red, Green and white. You are shown a random selection and have to name the top and then botton colour - eg Red over Green. Again you are not allowed any errors, but if you make a non critical mistake eg name green as white then they will do the test again in the dark, but with a very much reduced hue. Thre is a lot about the tests in this section so I won't go further.
Should you fail these tests at Gatwick they will class you CP4 colour unsafe. You can still get a class 1 medical and CPL, but its limited to no public transport and no night. You can however instruct and do glider towing etc, provided you do not have paying passengers or freight.
There are another two CV tests that the JAA and thus the CAA should accept. These are the spectrolux test in Switzerland and the anomalascope. Be warned about the anomalascope, it is the daddy of CV testing (apparentley) and a lot of colour normals cannot pass to JAA standards.
Hope this sheds some light mate and good luck.