ATCOs now have to pass a UK class 1 medical at Gatwick (not JAA class 1). The Swanwick one is more a pre-employment medical, and gives them a good idea that they won't be wasting their money sending you to the CAA later! When I went for my medical in Swanwick, the NATS medical folks were concerned that the CAA may have an issue with some medication I had been off since Feb, they were unsure if I had to have longer off them, so rather than waiting till I was at CATC, they got me straight to CAA to get it cleared, which they did!