In case you don't already know about it, the CAA's RT guide is here:
CAP 413 RT Manual
This is the official guide to what you need to say. The single most important piece in my opinion, is the reply to "Pass your message". You'll have to use this a lot.
That's where to find the answers. How to learn quickest? I'd suggest finding a pilot buddy if you can, and go flying together. One does the handling, navigation and command decisions, and the other does the radio work. On the return flight, swap around.
This will mean that when you are doing the R/T you'll have more time to think about it, and concentrate on it. You'll take it in much quicker that way.
When flying, you'll have to chance to listen out and hopefully learn a little from someone elses r/t or if not that, then at least from their mistakes!
dp