Maybe make Celtic traditions stronger in England proper by avoiding the Saxon invasions? That would give the Celtic Church a major urban center in Londinium to be based out of, which could give it more political prominence regionally. Other than that, there were already some changes made to early Celtic mythology to Christianize it; all that would have to be done is for these Christianized myths to be accepted as canon by the local church and then for said local church to split off from Rome.
I think
@CountPeter is interested in this stuff, maybe he'll have some better answers.
Or, for more fun, have rome not conquer britain, but have a native celtic kingdom in Kent or something modernize off of rome. This keeps the urban areas that would come with empire as celtic as possible-oh they'd liken themselves Roman's in the way the Frank's did, but they'd remain distinct.
Further, this makes it so Britain doesnt lose the warrior culture of the celts, preparing them to ward off the saxons better.
Other than that I only have two things for this tl:
No catholic england means no crusader England, which might stop independent Portugal, which has serious butterflies across the age of exploration
And the pope is going to be far, far less of a kingmaker, by the nature of a western church and an eastern church existing alongside the Catholics, so we could wind up with further splintering. No designated "reformation," but we could see a "Hispanic Church," or something if spain ever gets into enough of a tiff with Rome, or if the Celtic British are more useful in the reconquista than I originally gave credit to.