shgsaint,
When you go to Cardiff make sure you are only tested with the Ishihara plates and the Beynes and Holmes-Wright Lantern Tests. These are the only tests used at Gatwick.
Also, ensure they test you in accordance with JAR protocols and if possible that you are tested at the correct distances and reflected in a mirror. It may sound daft but that is how you will be tested at Gatwick so as much replication as possible of the CAA tests as you can possibly manage will mean that you are not filled with false hope.
If you can, print a Class 1 extended eye examination form off the CAA web-site and take this with you. Get them to complete it and take it to Gatwick with you but ask them to specify that the tests were completed in accordance with JARs. Can't say it'll make any difference but nothing ventured nothing gained.
HTH
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