Well this is why it's more of a smear vs distinct division problem. There were different languages yes, but there was also Common-Brittonic, and later Old English.
So yes you can divide these up into smear groups but also they were mixed significantly enough to also maintain a single shared cultural language across Britain.
If language and cultural history are the definition of ethnicity then these peoples are simultaneously two ethnicities, British and their own sub-groups of gaelic/brittonic/and further subdivisions depending on which point in the history of the island you want to look at and dig into. Like I said to the other user, the clearest and least blurred ethnic distinction historically is probably the anglo-saxon settlers at the specific time of their settling.
Aren't these literally the key factors in defining ethnicity. Are you under the impression that ethnicity is purely genetic or something?
Well this is why it's more of a smear vs distinct division problem. There were different languages yes, but there was also Common-Brittonic, and later Old English.
So yes you can divide these up into smear groups but also they were mixed significantly enough to also maintain a single shared cultural language across Britain.
If language and cultural history are the definition of ethnicity then these peoples are simultaneously two ethnicities, British and their own sub-groups of gaelic/brittonic/and further subdivisions depending on which point in the history of the island you want to look at and dig into. Like I said to the other user, the clearest and least blurred ethnic distinction historically is probably the anglo-saxon settlers at the specific time of their settling.