A connection between Lancelot and any Celtic figure would be made
possible, if still not plausible, if Lancelot actually could be found in
a 'Celtic' text. He's not in any of the Mabiniogion tales, nor part of
the Ulster Cycle (or any Irish Sagas) nor in any of the Saga Englynion from
early Wales. Lancelot does not appear until 'the Matter of Britain' had
become a European literary tradition, so to equate him with something Celtic is
just not on.
Mark Handley, Trinity Hall, Cambridge.