I think that the variations are such that it is actually impossible to know precisely what a person's surname was at any one point in time prior to more modern standard forms.
It is not rare to find a person, or a place-name recorded in two different forms in the same medieval document. The name Coombe, Coombes, Combe or Combes almost certainly derives from the Anglo-Saxon for valley. Yet another surname that is based on a physical geographical feature, and like the huge number of place-names which include it, therefore quite common. Other variants such as Coomber, probably relate to a dweller in a valley.
The Brythonic word for valley is cwm, pronounced as coombe. It is doubtful that these two languages invented a word for the same feature with such similar phonetic characteristics, so I would guess that one borrowed from the other. I would further guess that it was originally Brythonic because of the absence of any similar word in the areas of northern Germany, Schleswig-Holstein, Denmark, The Netherlands or the Frisian Islands whose languages share a common root with AS.
By contrast there are dozens of Welsh place and river-names with cwm as a component.