Britons at the time of the invasion of the Romans were divided up into Celtic tribes, such as the Brigantes in what became Northern England and the Iceni in Norfolk. They spoke the Brythonic language, the ancestor of Welsh. After the AD 43 Roman invasion under Claudius, some Britons allied themselves with the Romans and others like Caratacos fought hard against them. Prasutagos of the Iceni was one that allied to Rome, but when he died the dreadful treatment that his wife, Boudica and her daughters suffered at the hands of a Roman procurator caused a revolt to erupt and Camulodunum (Colchester) and Londinium (London) were amongst the cities destroyed. Boudica’s revolt was brutally suppressed.
Most Britons were Romanised in the centuries of Roman rule. But those in Wales and those beyond Hadrian’s Wall were less so, and they formed kingdoms in the early Dark Ages such as Gwynedd (in North Wales) and Strathclyde (in the west of what later became Scotland). The Anglo-Saxons took over much land from the Britons, and Old English replaced Brythonic (which we can start calling Cymraeg or Old Welsh) in the lands that became England.