First, there is an huge difference between linguistical features (that no one really cared about, critically before the VIII, when the germanic languages just began to differanciate from each other) and sense of belonging to a same continuuous entity : that could been underlined by the overlordhsip of various kingdoms over the smaller ones.
When Offa takes almost all England under his domination, he didn't titled himself "Great King of Mercia" but ruler of, IRRC, "southern England".
The same can be said for celtic high-kingship in Ireland or Celtic Britain.
It's a form of political union, and the expression of a sense of a common territory beyond kingships.
Before Germany and Italy got really divided up by feudalisation, you had such a sense of continuuous entity (as soon they could, Germans renamed their kingdom from east francia to "teutonic land").
Merovingian kingdoms are a sign of that as well : while divided, the fact that they represented parts of a same entity lead to continual tentatives of unifying the whole.
Anglo-Saxon England does not escape this. Its kings tried to unify what they percieved as a common ground.
Of course, when identity is defined by religion from one part, dynastic features from the other hand, and that your land is going all mosaic, it gets harder to enforce that.
A second point would be that England was underpopulated : maybe one million inhabitants, when continental regions had easily the triple, more than often quadruple, and more. It gets hard to divide 1 M into several little kingdoms without someone (one of the said kingdoms or a neighbour) simply taking over.
Finally, the point about Spain : as long Al-Andalus was a serious threat, you did have truces or agreements between northern christian principalties.
There were exemple of the contrary of course, and more than often succession crisis were such (in fact, some sides actively asked for cordoban help at one point).
Still the capacity of kingdoms issued from Asturias to reform into one entity disproove your point : at some times you had up to 4 principalties in the same time but as other exemples quoted, still concieved as a whole, and eventually unifying.
The infighting between entities other than sucession crisis (aka struggle, for getting the whole thing), really began when Muslim Spain was less of a threat and actually had a totally defensive position : Las Navas de Tolosa was an union of Christian kingdoms and feudal entities but the victory, breaking the last real threat against them eventually lead to their division.
In the same way, you could have the exemple of Mercia that didn't seem to have refused an united front with Wessex in the X against Norses. Granted, it was in their interests probably more than the sense of sharing a common ground : but nobody said absence of division had to be only a matter of being kind.