Brythonic polytheism refers to the cultural and religious practices of the Brythonic-speaking people of Britain.
Indigenous Celtic people that lived in Britain from the British Iron Age until the Middle Ages, when they split into the Welsh, Cornish, and Bretons (among others). They spoke the ancestor of the modern Brittonic languages, the Common Brittonic language. Now a Brittonic Polytheist would worship a wide range of god/desses from Roman Britain to later tales from the Welsh as within these tales there is an oral tradition. They also share in a lot of god/desses from Gaul. There seems to be a lot that is shared between the Brittonic Peoples and the Gauls.

There is a lot of overlap between Gaulish and Brythonic polytheism, which is partly due to historical trade, intermarriage, and cross-migration between the two sects. But they are not the same.

Now Brythonic polytheism includes Welsh, Cornish, Breton, and Cumbric polytheism. The last two I know nothing about, so if one has sources and all that, let me know.

Taranis lightning, the sky, rain, truth, strength, order, protection.

Coventina A tutelary goddess of the river and spring.

Nemetona was the goddess of the grove or sacred grove.

Brigantia is the goddess of war, healing, water fertility and prosperity.

Nodons a god of healing.

Cocidius goddess of hunting.