Hi there
From what I have learnt from these pages, I think you will need a UK class one anyway, so I think you are best off trying at Gatwick first, and then if you fail that then try in another country, because you will have to convert it back to a UK one anyway, so from what I understand you have nothing to lose by going to Gatwick first. I stand to be corrected, but I don't think you should have too much trouble converting from say a French or German class one to a UK one.
What's more, you could go to City University in London for a thorough colour vision test using lanterns that I think are used at Gatwick, because it may be helpfull to get accustomed to the procedures to prepare for the real thing, experiencing the different shades of light.
Good luck
