The CAA will NO LONGER change a JAA Medical from another member state purely on the basis of having passed that member state's colour vision tests. They closed this loophole several months ago.
They will now only issue you with a JAA CPL including the unrestricted Class 1 medical if you have already obtained the CPL in another member state.
So if you fail the CV tests at Gatwick and pass them in another JAA member state you will also need to undertake the ground and flight training in that member state and get a CPL issued by that state - only at this point will the CAA consider issuing you an unrestricted medical. Once you've gone through all the hoops of obtaining the CPL from another member state all you're doing then is changing State of Licence Issue to your country of residence.
If you go to another member state first, pass the CV tests then try to change it to a CAA issued medical they will still put you through the extended Class 1 medical which includes CV testing.
Sorry to be the bearer of more bad news but I had the info directly from the horse's mouth, about 6 weeks ago.