> The modern British nation is culturally descended from the post-Roman Anglo-Saxons, not the Celtic Britons who inhabited Roman Britain.
Whoa, that's controversial!
Some English people consider themselves Celts, and some consider themselves Anglo-Saxons. I'm not sure whether it's political or more like being a dog person or a cat person.
The Celtic faction got quite a boost in the nineteenth century, which is when a lot of things that people today think of as Celtic ("Celtic art", for example) were rediscovered, or perhaps, in some cases, some would claim, invented.
Celtic influence in the English language has perhaps been rather ignored: English words that seem almost identical in form and meaning to Welsh or Cornish words have had their etymologies given in dictionaries as "unknown" or "obscure". The distinction between present and present continuous ("I eat" versus "I am eating") is perhaps an example of Celtic influence in English, and it suggests that English was at some point widely spoken by people whose native language was Celtic.
(The Americans' failure to distinguish correctly - from the British point of view - between "I ate" and "I have eaten" is likewise arguably due to American English being at one point widely spoken by people whose native language was German.)