Well, first, if you want to make Alternate History, you'll need a Point of Divergence. I suspect you may have confused what it means, so allow me to explain and don't mind if it wasn't the case : a PoD is the event (or the series of event) that allows history to follow a different direction.
Without PoD, well, you have what we call Our Timeline, aka real history.
Asking for no Islam is in itself calling for a PoD.
Now, about your query.
Britain and northern Gaul (the distinction between north and southern Gaul appearing really only in the VIII century) being already Christianize (at least, where it counts, in urban and power centers), I don't think you can maintain them pagan when the WRE is no longer.
Furthermore, germano-romans kings finally converted to catholicism (whatever pagans or arians) cause not only the population was such (as well part of german people already) but because the local nobility and therefore adminisrtative power (they didn't replaced) was.
Finally, the association made between the king ruling over the subjects, in parallel with God ruling over Christians, certainly was attractive for rulers willing to be less dependent of a tribal institution that limited greatly their power and forcing them to share it more widely.
That said, how to make northern Europe more pagan?
Your first issue is that Mediterranean and western Europe was more rich and more inhabited than North. You need to make it less dynamic than OTL in order to prevent OTL conversion of Germany (that made conversion of Scandinavia pretty much unavoidable).
The best way is to prevent Frankish rise. For that, no Islam would help (as it gave room and legitimacy to Pippinids and more or less gave them Aquitaine and Burgundy) but also (and it's indeed counter-intuitive) christianize Frisians.
Without real legitimacy on taking over Frisia and destroying the takeover they had on North Sea trade, Franks would have harder time to expand themselves north (while they would certainly go eastwards, taking over southern Germany).
Another PoD needed would be to severely crush Byzantine Empire. That's gonna be hard without Islam, but let's assume Sassanids manage to win the war in the VII century. It could help to slow or butterfly the Christianize of Volga's basin, and an alternate conversion of Scandinavia.
But I don't really think it would be enough to prevent appearance of Christianism in Northern Europe. It may delaying it, allow a survivance of paganism at best, but...