You could say that Jago IS a form of James because both names come from the name Iacobus - which is also a name that Jacob is related to.I adore the name Jago. In Denmark we would pronounce it YA-go and it would, sadly, be unusable because it's the name of the parrot from Disney's Aladdin (though I believe he's spelled Iago in English). I think Jago looks better, though.
It is not likely that the "Anglican" source of it has a link to Jacob. As is there is strong evidence in Celtic "lore" that some form of the name was in use prior to the Roman invasion before the birth of Christ and the possible exposure of the tribes to Jewish or Christian tradition. One source even says that an early Roman governor made a "JAGO" a king of a portion of what is now Cornwall as an appeasement so that the tin ore could be mined without tribal objection. That may or not be true of course, as little, if any, written documentation exists from that period.It however should be remembered that other sources than England for "Jago". These are mainly derivations of central European names as they were Anglisized in the Americas. These may well have a Jacob derivation as noted.