When it changed from its pocket book format to CAA house style (the blue binder) it was claimed they could save paper by not having to publish it annually and could issue page amendments. They have never issued anything other than a complete new edition.
The document was originally coppied from ICAO Doc 9432 which hasn't changed wheras CAP413 is now on Edition 23. All the commercial exams are based on Doc 9432 not CAP 413. They have incorporated CAP510 and CAP452 into it.
Over the years it has become less and less user friendly.