As Britain was quite firmly rooted in Christianity by the 800's AD, do we know of any groups or sects who advocated against violence in the ensuing battles and wars on religious or moral grounds? I understand that Christianity was a different beast back then, but I wonder if there were any whose interpretation of the gospels would have rendered them pacifist regardless of the consequences?
In short answer, no: at least almost no such a 'pacifist objection' found in contemporary sources.
The closest parallel would be to leave the church institution with their remaining treasury, such as relic of the saint, richly illustrated manuscript and so on, and to wander around to find a new settlement relatively free from the Vikings. That is the what the monks originally in Lindisfarne (famous as the 'first' target of the Viking attack in 793) did after the repeated assaults of the Norse raiders in the 870s, with their patron saint, St. Cuthbert and some of his artifacts. Ironically enough, the local ruler who granted a new land to them to settle in from ca. 883 to the end of the 10th century was former Viking chieftain Guthfrith in Northumbria (Cavill 2001: 36-39). We can find some parallel episodes in the hagiographies in NW France about the same period, but a recent study suggests that the serious threat of the Vikings in such texts at least in France was sometimes employed as an excuse discourse (topos) to hide the more nuanced reality, like the theft of the relic from a coastal monastic community to another (Lifshitz 1995).
Europe around ca. 800 was far more 'violent' society than generally assumed, and we should not forget that the church itself constituted an element of such a society. It was not only the Vikings, but the Christian armies often assaulted the ecclesiastical estates, and the Christian rulers of the Franks also sometimes confiscated the property of the Church like the land in a not so entirely pacifist way (Reuter 1985). Some scholars even argues that there was not so much qualitative difference between the violence caused by the Viking raiders and that of the contemporary Christian plundering (Coupland 2004).
On the other hand, the Irish church, well integrated into the fragmented local politics of petty kingdoms across Ireland, had been both aggressors and victims of the violence well before the large-scale arrival of the Norse raiders (Downham 2018: 91, 138). The supporters of two Irish powerful church centers, Clonmacnoise and Birr, fought each other to compete for their territorial lordship in the end of the 8th century, and these churches could without doubt arrange their army against the new enemy, the Vikings. Downham summarizes as following: 'The individual trauma inflicted by viking raids should not be downplayed, but their impact on the Church, as a whole, is less than clear. From the 840s it was not unusual for vikings to work in concert with Irish allies to attack the churches of their political rivals......It appears that economic and political strategies went hand in hand, regardless of religious scruples' (Downham 2018: 139).
References:
Not regarding the wars against pagan peoples themselves i.e. Charlemagne's Saxon Wars (772 - 804), the defensive wars of the Carolingian kings of Francia against the Vikings (c.810 - 911) or the struggle of the kings of Wessex to roll back the conquests of the Great Heathen army (878 - 958). Ever since the time of St Augustine in the early 5th century, wars were seen as an earthly necessity for a fallen humanity tainted with original sin, and defensive wars against those who threatened the church were seen as undoubtedly just (Augustine wrote in his magisterial tome "In defence of the City of God against the Pagans" that God had willed that Stilicho defeat Radagaisus, an Ostrogothic warlord who still sacrificed to "demon Gods", by which he presumably meant Odin, Thor etc, near Florence in 406 but had allowed Alaric, a Visigothic convert to Christianity, to sack Rome in 410 just as he had previously allowed Carthage, Greece and Jerusalem to fall to the Romans). And then there was the Old Testament, in which King Saul loses God's favour for not slaughtering the gentile Malochites, and the Book of Revelation, in which at the time of the apocalypse Christ descends from Heaven with a sword between his teeth. Charlemagne's wars against the Saxons and Alfred the Great's wars against the Danish Vikings a century later were thus able to hold the status of holy wars (though it would be anachronistic to call them crusades), and when churchmen held any objections it was not to the fighting itself but to some of the excesses that came following victory in battle, particularly conversion at the point of a sword. Pope Leo IV, who organised counter-offensives against the Saracen pirates attacking Rome in the 840s, once proclaimed that soldiers who died fighting against heathen peoples would receive a heavenly reward - an idea that would from the time of the First Crusade in 1095 - 1099 evolve into the Crusader's indulgence.
It was with regard to fighting against fellow Christians and the celebration of the warrior as a vocation in Christian society that theological tensions existed. There isn't much controversy about this for Anglo-Saxon England since so many warrior epics like "Beowulf" and "The Battle of Maldon" have survived from that period, but for Carolingian Francia it is an area of a lot of debate, especially since it all ties in to the great debates regarding the "feudal revolution", the Peace of God Movement (attempts by churchmen from the 990s to limit the violence being committed by the "new" warrior aristocracy following the collapse of the Carolingian monarchy) and (in the very long term perspective) the origins of knighthood and chivalry. Janet Nelson in one of her essays in "The Frankish World 750 - 900" and Karl Leyser in his essay "early medieval canon law and the origins of knighthood" (compiled in "Communications and Power in Medieval Europe: The Carolingian and Ottonian Centuries") have argued, both from legal codes and the histories of Nithard (a maternal grandson of Charlemagne who was both a soldier and chronicler writing in the 840s) that being a warrior was actually a very crucial part of being an aristocrat, whilst continental historians like Georges Duby in "the Chivalrous Society" and Arno Borst have argued that the warrior nobility was a post-Carolingian phenomenon and that most Carolingian dukes and counts would have essential seen themselves as noble civil servants who would have followed churchmen like Hincmar of Rheims, one of our principal sources for the 9th century, in viewing the shedding of Christian blood under any circumstances as homicide worthy of severe penance and the profession of the soldier as a low born one. As y_sengaku has pointed out, Timothy Reuter "in Plunder and Tribute" and Paul Fouracre in "Charles Martel" have pointed out that Carolingian rulers relied heavily on a warrior following, rewarded with a constant flow of land and plunder, for maintaining their hegemony and had little problem with expropriating from monasteries and bishoprics when they deemed it necessary in order to reward these followings of soldiers, not all of which would have been low-born retainers. Indeed, Reuter has argued that if the Frankish epics had survived in written form we'd be able to see Carolingian society and its attitudes to violence a lot more clearly, because the records that do survive - royal capitularies and ecclesiastical chronicles do cast quite a distorting mirror on the period. I may have gone off on a bit of tangent here - I was just trying to outline the historiographical debates on Early Medieval Christians' attitudes to violence.