Hello, I answered this question previously here and also touched on it here too.

On top of those answers, it's interesting to note that when Æthelstan mints his Rex totius Britanniae series of coinage following his victory at Brunanburh, the coinage minted at the former Danelaw stronghold of Derby still labels him as Rex Saxonum, illustrating that perhaps there at least, he was seen as an imposition.