Ahhh, I understand. At City I did one run lasting rougly 10-15 minutes. I wasn't given a choice...whereas at CAA Gatwick I did the fast CAD (2 minute screening...apparently its done for people who panic when reading the Ishihara plates and make errors who are actually colour normal...that's how it was explained to me). Then if you fail that you do a full CAD and either pass, fail or get borderline. If you get borderline (as I did) you have to repeat the full CAD 2 more times and then they average your result.
I'm not sure whether you could ask to do more runs at City, I suppose it wouldn't hurt to ask if you wanted to do a few in one session...the woman who conducted it was nice and friendly.