As mentioned, CAP413 is the definitive document. NATS are very particular about RT discipline and have recently added the requirement to say the word "degrees" after a heading instruction except if it ends in a 5 (eg "turn left heading 270
degrees"). They also require the word "altitude" in climb/descent instructions, eg "descend
altitude five thousand feet".
CAP413 1.4.4 clearly says
All six figures shall be used when identifying frequencies irrespective of whether they are 25 kHz or 8.33 kHz spaced. Exceptionally, when the final two digits of the frequency are both zero, only the first four digits need be given.
The CAA sent all commercial pilots a booklet earlier in the year entitled
CAP 413 Supplement: A Quick Reference Guide to UK Phraseology for Commercial Air Transport Pilots and I understand there is a plan to eventually issue similar for private pilots.
If you are starting out, then there is only one method to learn. Plenty of accidents have been attributed to poor RT discipline. If the book is wrong, it needs correcting, not for you to start your flying with incorrect habits "because it said so".