UK ATCOs now do the European Class 3 medical. This is what it says on the CAA site with regards colour vision...
Colour Vision Requirements
UK CAA colour vision requirements for European Class 3 (ATCO) certification
Colour Vision
An applicant for a European Class 3 medical certificate must have normal colour vision (normal trichromacy).
An initial applicant must make no errors when tested on the first 15 plates of the standard 24 plate Ishihara Test (a series of numbers outlined by different coloured dots, easily seen by someone with normal colour vision).
Those failing the Ishihara test will be referred to the AMC for secondary testing.
See
here and
here for more detail.