Hi LFG,
CAP 413 covers everything you need and some more as well. You should be able to guess which bits you won't need e.g. you won't be asking for start and pushback with a training aircraft. You will, however, be expected to know the proceedures at an airfield with full ATC, i.e using callsigns GROUND, TOWER, APPROACH, (RADAR), (DIRECTOR). You will also need to know the difference between full ATC, AFIS (callsign ....INFORMATION), and AIR/GROUND (callsign ....RADIO).
You also need to know what has to be read back in full, a full list is in CAP413. The way in which numbers are pronounced when they are 3 or 4 digits should also be familiar, e.g. tree tousand fife hundred feet, not tree fife zero zero feet.
For the practical test you will have to ask for a MATZ penetration, and either give a Mayday call or relay a Mayday call. One of the things the examiner looks for during the practical test is that you don't press the press-to-talk button if you hear another aircraft give a Mayday call even if you have just been asked for a report. On hearing a Mayday, all other stations cease transmitting until advised by the controlling station. The exception is if the ground station obviously hasn't heard the Mayday (perhaps weak) in which case you relay the message.
I have done this for real and its much harder than you think. Write down everything important you hear before trying to relay it or you will make mistakes.
If you are nervous, why not ask the examiner if you can do a mock practical test first.
Good Luck.