So what if for some reason the Vikings never establish anything like the presence they had OTL .Whether this be due to stronger Mercian resistance or troubles at home .
Wow, that is almost as difficult to answer as "What if for some reason the Romans never conquer Gaul?" The changes are immense.
Do other Viking still go to Ireland? Do they go to the Channel coast of West Frankia, aka Normandy?
It is difficult to answer beyond this:
+ the Mercian dialect of Old English will become more influential in the formation of Middle English
+ If the Mercian primary residence remains at Tamworth or generally in the area of Greater Birmingham, that will lead to a much greater distance from the City of London (as nearly inevtiable primary trading port) than if they resided in Westminster.
+ But unless no later invasion succeeds, this might be a long forgotten thing in this TL's past as well.
Great Britain is not so small an island that it has to be one dominant country where England is and then Wales and Scotland. Over time, there might be a defiant Aenglic kingdom in Mercia, Cymric Celts in Wales, Gaelic Celts in Scotland, but also an Armorican French speaking nation between the Channel and the Upper Thames and perhaps a Low Frankish speaking nation of Flemish settlers in Sussex and Kent. Just because a Midlands-based Aengland could not expell the incoming settlers quickly enough.
It is hard to say.