During the migration period, have Slavic peoples play a similar role to Anglo Saxons in Britain, or at the very least have them leave lasting Slavic imprints on English
Historically, the advance of Slavic peoples in Central Europe is essentially due to the partial desertion of these same lands from the IInd century onwards up to the Vth by whole groups of peoples that more or less slowly gathered at the Roman border, forming new set of peoples with natives (Goths, for exemple). regions that weren't particularilly strongly inhabited either.
In these peripheral regions, there wasn't much demigraphical rivality or polities strong enough to either repel them or acculturate these.
But, for reaching Britain's shores, Slavic people would have to move very quickly to North Sea, which gives some obstacles.
The first being that Slavic migrations generally followed a familial pattern, rather than whole peoples as most Barbarians in the Vth (which beneficied from a large integration within Roman institutions already, and the fact they were litterally at borders and on both sides), which provoked a relatively slow advance. It changed a bit due to the Avar hegemony over several Slavic peoples, gathering them around it as chiefdoms, but it wasn't really doable before Slavs really managed to differenciate themselves and before Germans and Sarmatic Barbarians got off the way.
And even arguing of a super-complex chiefdom as Avars or Bulgars, as much Pannonian Plain and Lower Danube are fit these large ensemble...Germanic Forest and Alps are rather more problematic, so best leaving this precise idea.
Then, Slavs would have to deal, if they can't advance quickly, with the post-imperial Roman world, and its successor in western Romania, Franks. By the time Slavs showed up on their doorstep, Franks were quite powerful and tried their best to raid and enslave them (which is why we call slaves....slaves and not a derivative of servi).
Note that I could see, without real problems, Slavic peoples from the Avar sphere of influence leaving it
and forming sort of a foedi inside Francia or at its periphery. It did happened IOTL in Italy. But I don't expect a full-fledged migration.
You could argue that Slavs could try to pull what they did in Balkans, having a PoD forcing a weakened Francia in the region (which shouldn't be about Francia divided stuff, because this tended to reinforce the military capacity) : it's not that plausible for several reasons but not impossible either. At this point, tough, one is kinda forced to wonder why Slavs would go beyond the Channel. It's poorer and more of a mess.
And then, ignoring or having succeeded dealing with what is briefly mentioned above, is how Slavs behave when in Britain.
In the Vth century, the general desintegration of power led Germanic communities to follow a familial migration pattern.
In the VIth or VIIth, however, Polities are much more well definied, with set identities and with a complex enough organisations : the process of migration/acculturation of Germans (from Germany and Scandinavia) into petty kingdoms wasn't really doable anymore and would have required a more military approach, like Danes and Norses did in the VIIIth to XIth century. Which isn't really what was about Slavic migrations : again, I could see these as groups settling, but not forming enough of communities or political centers to be really this relevant culturally.