What was the name of the druid leader at the time of the roman invasion?

Answer 1

There was no "one" Druid leader mentioned by name, though Julius Caesar mentions the existence of a head Druid; note however, that this is in reference to Gaul (in his Gallic Wars), not Britain as you most likely mean ("time of the Roman invasion" is terribly ambiguous, both as to date and geography, as there are atr least a couple options for each combination). We have almost no details about this class of peoples, except for brief mentions by writers like Posidonius, Caesar, and Tacitus. Posidonius separated the special upper class into three groups: bards, vates, and druids. The vates, according to Posidonius, were the seers, nature studiers, and overseers of sacrifice, while the druids were more concerned with law. Caesar combines the two levels, describing a priestly upper-class concerned with sacrifice and religious rituals, but also committed to memorization of all knowledge, incluuding natural history and law. Although describing the situation in Gaul, Caesar does say that most think Druidism originated in Britain. However, most details actually come from Gaul, and most Roman accounts merely quote Posidonius in their details, notably Strabo, and even Caesar had clearly read his accounts.