Here's what the CAA say about the subject. Section 4 describes donning and adjustment. Therefore I'll suggest that UK registered airlines, quite reasonably, have assumed this to mean that a "double knot" is an acceptable method to secure a life jacket. I will not argue against that. But there again, so is any other non-slip knot as long as it is quick to tie. What this thread shows that cabin crew are not given sufficient background regarding the origin of their companies' procedures, as they should be. Failure to supply the relevant information means that invalid, incorrect and at times ridiculous reasons are invented to explain why we do what we do.
PM