The main benifit in getting the ticket issued in Canada is the exclusion of the "higher fare" rules.
i.e. Get a JNB priced in originating DONE5 issued in Canada for the ZAR45,600 base price (~USD6000).
Certainly not such a benefit for an ex UK fare, but it is there. (GBP5169 or ~USD10,200 against CAD11,400 or ~USD11,500)