Tea and Crumpets and So Forth

Question: Are there any Legend 12 gods that are exclusively English? I know Britannia doesn't count and isn't Legend 12, but I'm interested in what associations she has.

Alas and indeed, Britannia doesn't count. She's a neat lady, but while she's the goddess of Britain, she is not actually British herself, being a thoroughly Roman deity. You're also right that she's not Legend 12, since the Romans mostly personified her to represent the territory and to have something to show their own gods and emperors as superior to, and there are no surviving myths about her other than some images of her being conquered by Mars or Caesar. She probably would have vanished from everyone's consciousness almost completely if Elizabeth I, that sexily badass paragon of queenliness, hadn't spearheaded her resurrection in the sixteen century as a symbol of the power of England, and since then she's been on a lot of coinage, stamps and other official symbols of the country's government even unto the modern day. She's now basically the Uncle Sam of England, though of course more antiquitious and Britishly fancy, and her origins as an actual divinity have been lost far into the mists of time.

All that is interesting, but it doesn't give us much to go on in terms of associated powers for her. We would assume she's Legend 9 and has been for a long, long time, and that her most likely association is War, since she was usually shown as fighting the (losing, but give her props for trying) war against the Romans when she first appeared and later represented British military and naval might and colonization. She's a good example of a goddess who came up sort of out of the blue and might have been a Scion herself, most likely of Athena, to whom she's markedly visually similar.

As for exclusively English gods... I'm afraid I have to disappoint you, because there aren't any, or at least not in the way we tend to think of England right now. The British Isles were populated by an ever-shifting murk of Celts and Norsemen, their influence waning or waxing in different areas depending on the current wars and rulers, and thus the evidence in England's ancient times all points to worship of the gods of those people. The Saxons brought the Norse gods with them, leaving traces of Odin, Thor and Volund before Christianity swallowed most of the island whole, and the Celts who predated and coexisted with them worshiped the Tuatha and the gods of Wales, leaving evidence of Lugh and Nuada and the Dagda and the rip-roaring tales of the Cath Maige Tuired and Mabinogion as well as infrequent shrines to other Celtic divinities such as Cernunnos. England wasn't considered its own cultural or ethnic group for a long, long time, and during that time it was pretty much a neverending party of northern European gods meeting at the crossroads between their territories.

And, of course, there were Romans in southern Britain, so the Dodekatheon had their say as well. If you're looking to play a historical game set in the wayback times, you could probably throw a rock in any direction and hit a religious observance dedicated to at least one of these three sets of gods. If you're wanting to play the most English Scion possible, my personal recommendation would be one of the Aesir, since the Anglo-Saxons became the basic stock of Englishmen until the Norman invasions, but the Celtic gods are equally reasonable choices.