You'd have to somehow make Gaul pagan, which is...very difficult, and way beyond my ability.
As long as a powerful Christian state is right across the Channel, one of the southern Anglo-Saxon kingdoms is eventually going to make alliances with it and welcome missionaries, and once that happens it will eventually spread to the rest of them.
Also remember that unless you butterfly St. Patrick you'll still have Christians in Ireland, and Wales will have some Christian presence regardless.
On the other hand, Theodosius is before the Anglo-Saxons even settled in England on a large scale, so if you keep them in their homelands they might last longer. But I'm not sure that's what you're asking for.
the conversion of the angelcynn was an obsession with pope gregory.non angli sed angeli -nots angles but angels)so to have even the slightest chance of keeping the angelcynn following their ancestral faith would require another man becoming pope which in turn creates a major set of butterfly effects.it would be possible to delay it for at least another couple of generations.an example would be to have penda of mercia defeat oswald of bernicia at the battle of heavenfielf (635 a.d).the best you can ultimately hope for though is for the church to incorparate even more of the traditional heathen beleifs practices and festivals then they did in OTL.
What is the best, most plausible POD after the death of Emperor Theodosius for Anglo-Saxon England to not convert to Christianity and retain their traditional form of spirituality?
After Theodosius' death? Not a chance as well.You'd have to somehow make Gaul pagan, which is...very difficult, and way beyond my ability.
cadwallon of gwynedd was allied with penda of mercia at the time of the battle of heavenfeild.now as for norhumbria itself was made up of two sub kingdoms bernicia and deira oswald reunified northumbria after he defeated the mercian welsh alliance.and your correct christianity allready had a foothold in the english hepartachy.although none of the heathen kings converted untill after the arrival of the gregorian mission led by augustine(of canterbury).and i note that peada introduced christianity in653 a.d when augustines mission to king ethelbert of kent arrived in 597 a.d and ethelbert did not convert to christianity because of his frankish wife after all the franks and other christian lands still traded with heathen england because trade is trade(the same as christian kingdoms traded with the muslem caliphates even when they were at war with each other).the main reason various english kings converted was that the white christ was seen as a powerfull god but just because they had converted did not allways mean they had forsaken the old gods.we know king redwald of east anglia at least initially worshiped both the christian and the heathen gods.and the conversion of england was not allways clear cut because ethelbert(the first christian king)was sucseeded by his heathen son eadbald who drove the gregorian mission out of kent and returned the kingdom to heathenry.so while the eventual conversion of the english was probably inevitable in the long run its by no means a forgone conclusion that it would have happened at the same time or in the same way as otl.Penda of Mercia wasn't involved in the Battle of Heavenfield; that was a battle between Oswald of Northumbria and Cadwallon of Gwynned, both of whom were Christian. And obviously, Christianity was well-rooted by that point, with a number of Christian kings already established, including Penda's own son. Which is another issue; Christian rulers attempted to convert their pagan subjects to Christianity much more heavily than Anglo-Saxon pagan rulers tried to convert their Christian subjects to paganism.
But even if you kill Pope Gregory, Christianity is already spreading to England. In addition to the Celtic Christians, there are already close trading ties with France. One of the reasons Pope Gregory's mission went to Kent initially, was because the wife of the King of Kent was already Christian, being a Frankish princess. Marriage alliances were an important part of the conversion process, as the agreements generally included that a Christian princess marrying a pagan ruler keep her faith and bring a chaplain with her. Depending on the situation, the pagan bridegroom might have to convert as well, in which case it's in his own best interest to promote the new faith within his domains.
First you need to get "The pagan church", rather than what was [probably] a fairly loose collection of pagan (or 'heathen') priests.The pagan church would need to offer something similar to have any chance.