A lot of rock, punk, and metal music is heavily entwined with British music, even if not explicitly solely created here. Therefore, I am including these genres here (at least, the ones that are more closely linked to British music in some form), albeit they likely aren't primarily British in focus.
Below are communities that already exist in a more-active form listed above. They're listed here nonetheless for posterity, but the above-listed equivalents are probably better options for you.