They didn't react to them, because as far as we can tell, there were no surviving population groups practising pre-Christian religions in the British Isles when the Vikings first began making incursions in 793. Ireland and Scotland had fully Christianised over the course of the fifth to seventh centuries and developed pervasive Christian identities of their own right -- especially so in Ireland, where sharing religion and language across the many hundreds of small states across the island facilitated a degree of common ethnic consciousness that made Norse settlers initially quite unwelcome. When the Vikings began appearing at the end of the eighth century, there were no pagans left in the British Isles, or at least any remaining communities were small enough to escape documentation and to leave no trace in the archaeolgical record.

If you're interested in how Scandinavian pagans interacted with other polytheistic communities, it might be smart to direct your inquiries into their early voyages into Eastern Europe, especially the Kievan Rus. I haven't studied that part of the world to the extent I've studied Ireland (and to a lesser extent, Scotland), so I can't speak with any authority there, nor can I recommend you any texts.

Sources: Stefan Brink and Neil Price (eds.), The Viking World, various articles

J. H. Barrett (ed.), Contact, Continuity and Collapse: The Norse Colonisation of the North Atlantic, various aritcles

Clare Downham, Viking Kings of Britain and Ireland: The Dynasty of Ivarr