Just wanting to add an update to Eagle 1's question. I was up for selection a couple of years ago with the RAF but unfortunatly I failed the Ishihara plates. Even though I held a JAA Class 1 medical, they did not offer the lantern test.
I am not wishing to contradict legalapproaches answer but things may have changed since he took them. The impression I had from the medical officer at cranwell was that sometimes the requirement changes depending upon supply and demand.