Hello,
I posted a reply to a similar topic a while back. In it I described the KOCH method for learning morse. Honestly lads, this is one of the universally most effective ways to do it. Like Islander Jock I was a professional radio operator, albeit in a maritime capacity. I went through all the pitfalls in learning it and eventually realised that it should be learned so that it becomes instinctive, visual or other memory aids are counter-productive. Through realising this I eventually got up to around 45 WPM after a number of years. First rule, morse is not a code, it's a language. Now for your CAA exams the speeds will be so much slower than communications morse that it mightn't seem relevant but do yourself a favour and learn it the easy way, set aside regular practice times every day, 15-20 minutes should be enough. With the KOCH method you start off with just 2 characters and keep adding more as you go on, this is similar to the way children learns the sounds that eventually enable them to communicate. It's all about Rhythm. Whoever got the idea to present morse in encyclopaedia's as dots and dashes obviously never used it.
Check out the following url:
http://www.ees.nmt.edu/sara/sara/finley.morse.html
Good Luck.