When reading about Pytheas (and what remains of his writings about his iron age journey to Britain) , I saw that:
"Diodorus based on Pytheas reports that Britain is cold and subject to frosts, being 'too much subject to the Bear', and not 'under the Arctic pole', as some translations say."
What could that possibly mean? If it helps, on the Wikipedia page, the word Bear in that sentence links to the page for Ursa Major, perhaps specifying something that was evident in the original Greek text, but not in the English translation.
I believe you've answered your question in your question:
Britain is cold and subject to frosts, being 'too much subject to the Bear', and not 'under the Arctic pole', as some translations say."
The North Star, Polaris, is part of the constellation Ursa Minor or "The Little Bear", and two of the navigation aids for finding it are in Ursa Major, or "The Great Bear". This is a poetic way of saying that Britain is too cold because it is too far under the "Bear" constellations - in other words it's too far North.