You will need to study CAP413 thoroughly.
'R/T Theory' is a bit of a misnomer. Virtually nothing about radio theory, it's mostly about correct procedures and phraseology. So you need to have a reasonable understanding of things like Flight Information Service - and not to be able to calculate the length of a half-wave dipole element at aeronautical frequencies or things like that!
It's not hard - but you must study CAP 413!!