The same Boii had occupied classical Troy at the time of it's latest destruction and had moved on, in part to various places around the Mediterranean, as well as on the Adriatic.
They managed to smack the Romans pretty good a few times from their redoubt in Bologna. By the time Julius came along they were living in Bohemia and Switzerland. He rousted them out of there and sent them to Gaul.
My focus, however, is on the group that ended up in Ireland and then what is now England.
Obviously they needed a ceremonial center ~ and the Apollo reference (by Greeks) does make sense.
About a century and a half later they were allied with McWallace in Spain and Gaul against the Romans in the Punic Wars.
I would imagine the earlier Greek records might be all that's left of specific historical references to them by anyone who cared.
Some interesting possibilities here: Apollo was the God associated with colonization and with the Sun. He dragged the Sun behind his chariot on its ride across the skies each day. So a city dedicated to Sun worship might well be called a city of Apollo by a Greek visitor.
Apollo was also the God of colonization and was consulted on any proposed colony via the oracles. Greek colonization dates from 750-550 B.C. and so when Pytheas reached Britain and found an old (deserted?)city dedicated to the Sun god, he might well have dubbed it a ‘lost’ city of Apollo.