A friendly Andrew posted a reply to my message on a board at Ancestry today. Turns out, from info obtained in John's military record, that he entered the regiment from Worcestershire - not Lancashire. So at some point he obviously left the Preston area.
Because he was 21 when he was killed, there was thought that possibly he was married prior to entering the service - I have not yet found any marriage records for young John.
I've not been able to nail down marriages of all his Mother's sisters to this point. Their first names are quite common for that time and there were quite a few McCann family members in the area. I do know that several of them were still single in 1911 according to the census record, however after that date there are still many duplicates of the names, making it next to impossible to identify the correct unions until I am able to order marriage certs for each of them.
Another possibility is that John was left with his Father's sister - Frances Bellew, however Frances was born and died in Preston, never married and didn't have any children - I don't think it's likely that John would have left her completely alone and go to Worcestershire - but anything's possible.
So the search continues...
56 minutes ago