Hi short finals,
It doesn't really matter where or how many time you sit the CAD. If you fail the plates at Gatwick, then they will ask you if you want to sit the CAD. It's a lot more expensive to sit it there than at the Uni. Plus I found the screen they use at the Uni produced a clearer image that the one at Gatwick and for the price you pay at the Uni (It's a donation really) you get to sit all the colour vision tests that are currently available.
The only downside about the CAD results is that the CAA do average them. Roll on medical flight tests!!!
Good luck though!!