Your challenge, with a POD in 500(start of Slavic Great Migration), is to push Slavs into the northern British Isles. Southern Britain is also allowed.
Best chance would probably involve pushing Obotrites a bit further west to the Wadden Sea either around or just after Charlemange (say by primary PoD being failure from the Franks to push as far north), and then keep pushing (secondary pod) so they move across North Sea using the conflict there between the Anglo-Saxon kingdoms and the Viking Raiders, to get a foothold in some relatively remote and unimportant corner of the British Isles.
Scotland is doubtful since the Norwegian Vikings was based in that area, but they might be able to pull off Cornwall and Western Wessex
So the Norwegians were already powerful in Britain during the time?