Simple answer - yes. @davesparks explains in detail in post #2.
The four C2's are saying that RCD protection is required for a variety of reasons. (Some of them are in fact highly questionable and likely utter rubbish)
If every circuit has RCD protection then that is the remedials dealt with.
Sure but what one of the electricans said to me is its one thing finding one that may fit but RCBOs are absolute pain to install and work with due to high risk of tripping and they simply wouldn't touch RCBOs because they have too many issues down the line.