Siculus & The Temple of Apollo
I wasn't previously aware of Siculus's account of Pytheas's voyage. A suggestion that the British were worshipping Apollo at a significant Temple, is to me highly revealing - assuming the account can be trusted. Regardless of whether the site referred to was actually Stonehenge, it adds weight to the belief that the "British" had an organized and widely accepted religion in the 4th Century BC, and by extension probably much earlier. If it were indeed Apollo, although unlikely, would suggest much farther distant (in time) contact with Greek culture. Alternatively, if they were worshipping someone/thing similar in nature to Apollo, this would suggest worship of Apollo-type characteristics: the sun, light, oracles, knowledge, medicine/healing, music, art, poetry. A lot of IFs here, but I think this paints a neat picture of a people maintaining a site of pilgrimage (possibly Stonehenge) for those across the British Isles and Western Europe who were seeking wisdom, knowledge, oracles/divination, healing, or just cultural/technological exchange.