What your recommending sounds fine but you should be aware that with the change to separate unit exams in 2010 for QCF, there has been a slight shift in the emphasis of the syllabus - some things which you had to study for the 'old' exams aren't in level 2 Plant Growth or Garden Planning, other things are more important (e.g. sustainability).
A good example is that the process of secondary thickening is no longer on the level 2 syllabus ( it moves into level 3 in more detail) so you can't be asked questions on this - and there have been some changes in what you can expect to be asked in soil science too.
There's nothing wrong with covering wider topics for interest and depth, but if you are concentrating on passing the exams your key is to keep with the syllabus, whether you are on a course or not. I'm not aware that Dawson's book has been revised since the new syllabus came in - perhaps it has? Also HCC are a good outfit and will have flagged the changes in their course materials - because the QCF revision 'slimmed down' the syllabus a little, not all courses have adjusted their notes.
Reading the syllabus and identifying what the exact wording means, and how it guides you to the level of detail you can be asked to provide, can make a lot of difference in getting ready for the exams.
After this summer's exams my colleagues and I prepared some advice notes on studying and revising, and on the wording used by the RHS and how to interpret it. It will be freely accessible to any interested RHS students via our Facebook page - not just our own students - and I am happy to give the link to any support sites, so I'm going to put the link here:
however if this breaches the guidelines for the forum and we need to remove this I will be happy to resend it direct.
good luck with your studies,