Since “Anglo-Saxon cat” has not produced much results, I started looking at Celtic and Scandinavian legends.
In Northern mythology (H. R. Ellis Davidson, Gods and Myths of Northern Europe), the goddess Freyja’s chariot is drawn by cats (p. 30). In one particular story set in Greenland, Eiríks Saga Rauða, Freyja appears as a vǫlva (seeress) practicing seiðr (divination magic), wearing catskin gloves, white fur on the inside (pp. 117-120). When Thor and Loki travel to Utgard (p. 34), one of the contests Thor fails (due to magic tricks by the giants) is picking up a large gray cat (which is actually the World Serpent, so the fact that Thor lifted its paw was pretty impressive).
In Scottish legend, there are some fearsome mythical cats. Sidh is a large black cat with a white spot on its breast. Some scholars associate it with the large (43″ long) Kellas cat documented in Scotland, an apparent hybrid between a wildcat and domestic that has white guard hairs on the flank. Domestic cats are primarily descended from African wildcats not European ones.
So Aldred’s cat could be white, black, or gray. I could make his an unusual Scottish cat of large dimensions like a Kellas cat, but that might be a bit much. A contemporary British short hair, as above, could be a model.
And for something completely different, those in Northumbria could attend the children’s play at the Custom House in South Shields, Eadfrith, His Cat and Their Gospels. It features two cats, one of which is apparently a monk.