Just to elaborate, Brythonic was the language of the Celtic Britons. Welsh, Cornish and Breton all evolved from Brythonic. It wasn't the Romans who changed the language of England, however, but the Anglo-Saxons who came after them.
Technically there was no England as such before and indeed during the Roman occupation. We do speak of this country (i.e. Mainland UK which is was conquered by the Romans) as Britain however. The Romans of course did influence Brythonic but they didn't feel it was necessary to replace it with a new language such as the Anglo-Saxons did.
Just to elaborate further - brythonic was a P-Celtic language. Whereas, Q-Celtic was the language spoken by the Celts of Ireland, Scotland and Man. So where the Welsh mould say 'mab' for 'son', the Q-Celts would say 'mac'. It gets more complicated but that's enough for you.