Board Thread:Spelling debates/@comment-188432-20130428153100/@comment-26975268-20130428171633

Doctor Who DVD Files also uses 'encyclopedia' in all their cover headers.

So, in-universe, it's rather arbitrary, and, in the Who-related real world, 'encyclopedia' is by far the predominant spelling.

So there's two points for using 'encyclopedia':
 * 1) All Who reference books use it.
 * 2) It's recognisable to all, while 'encyclopaedia' might seem wrong to some readers.