I could see a Slavic Jutland, but a Slavic Britain is so unlikely that it's borderline ASB. Even in OTL it took a lot of unlikely things for Britain becoming Germanic, if the Roman Empire had collapsed a century later or earlier, Britain would likely be Romance or Celtic talking today. So let ignore that part.
Could Jutland become Wendic, yes it easily could. After large part of the Angles and Jutes had migrated the Danes moved fast in and conquered the remnant population, a crisis among the Danes could have left the Wends room to move in instead.
In fact Jutland north of Schleswig was prime territory for Wendic style farming.
If people look at this map:
The map show the soil quility in Denmark, bluish green is the best one and pale orange is the worst one (the bluish green green give 25 times the wheat as pale orange). Why is this important, because the Slavs with their more primitive agricultural technic prefered the poor soil, because a simple wooden plough could be used in the sandy soil, while the richer loom took a lot more labour and was hard for them to use. So they could relative easy set up a principality up in Jutland.
But in the long term it won't survive, but still it will likely change European history in many ways. First we will se Danes and franks come into conflict later, likely pushing the Vikings (as something more than raiders) if they happens into the future.
Second without a strong Danish kingdom in Jutland and only a weaker Wendic principality, the Frisian will likely settled the entire western coast of Jutland (the Limfjord would be a natural habitat of the Frisians, while the Wends would find it too swampy).
The Danes on the other hand may see the southern Baltic coast as a more natural place to conquer, as it's thinly populated and closer to the main Danish stronghold of Sealand-Scania. Denmark are also likely to have another name, as Denmark was likely original a term for a Danish border principality in Jutland, who later took over the islands (according to Saxo through marriage).
In Britain without the Danish invasions, there will be less push to unify, it will likely happen anyway, but it may happen in a more ad hoch way.
The Slavs on the Jutish heath, even if the Danes, Frisians, Franks or Saxon take over will likely survive until the 19th century as the Wends on Lüneburg heath did, as the conquer will be more interested in the more fertile earth coast and north.