The best (and only legal) source of info is CAP413 from the CAA
There is nothing legal about it!
1 Document Description
1.1.1 The aim of the United Kingdom Radiotelephony Manual (CAP 413) is to provide pilots and Air Traffic Services personnel with a compendium of clear, concise, standardised phraseology and associated guidance, for radiotelephony communication in United Kingdom airspace.
1.2.4 Candidates for JAA pilot and instrument rating examinations should note that the syllabus for the communications examination is drawn directly from the International Civil Aviation Organisation (ICAO) Annex 10 Volume 2 and ICAO Doc 9432-AN/925 and not CAP 413.
1.3 Document Source
1.3.1 The UK RTF Manual is based on ICAO Annex 10 Volume 2 (Communications Procedures) to the Convention on International Civil Aviation and ICAO PANS-ATM (Procedures for Air Navigation Services - Air Traffic Management) Doc. 4444.