I would certainly second the idea of getting a second hand unit cheap from somewhere (as long as it was digital, etc) as after you gain your PPL you may find yourself wanting to upgrade to an airband tranciever as a backup to the on-board radio in case of failure (or find yourself drawn to aircraft without an installed radio).
Don't bother with the major airport frequencies as they can be very difficult to follow, aircraft are often switching frequencies and they are not exactly CAP413.
Depending on where you live, you should find a number of airfields within easy enough reach providing a range of services that you can listen to that will help, however, as with most things, practice is the only real way of improving your own skills. Go through the "conversations" in CAP413 with someone else and also work through
Safety Sense Leaflet 22 "Radiotelephony" which also has an example of the RT practical exam in it.
BP