This looks like slang where the end of the word is replaced by "aggers". Some more insight is given in Fry's Moab is My Washpot:
"Wagger," or "wagger-pagger-bagger," which was used to denote "waste-paper basket," is an example of that strange argot prevalent in the 1920s and 1930s that caused the Prince of Wales to be known as the Pragger-Wagger.
Even today, in the giddy world of High Anglicanism in such temples of bells, smells and cotters as St. Mary's, Bourne Street, SW3, I have heard with my own two ears Holy Communion referred to by pert, campy priests as "haggers-commaggers" and my mother still describes the agony and torture of anything from toothache to an annoying traffic jam as "aggers and torters."
Oxonist mentions an -aggers suffix which is used in Quagger, Jaggers, Staggers and also in a slightly different form in Ugger, Wugger. Prince Michael (even if he's not the Pragger Wagger) attests "the Coronaggers".
Really, it's almost like some sort of pragger jogger.
Cross referencing everything, many of these words appear on Wikipedia's page for Oxford "-er", which means that's it's probably Oxford slang that spread. (Of the all the words, soccer and preggers are well known outside England.)
I have not found anything on this but the fact that Fry encountered the other terms through his day to day life means that I'm inclined to believe he's right.
The Green's Dictionary of Slang entry found by Gio gives solid evidence that "Jessica" is used to mean "Jesus" somewhere (apparently in the South African gay community) but I can't see how this would be related to any British or especially High Church usage.