Kalos's legendaries were based off of Nordic mythology and Hoenn's main trio were based off of Hebrew mythology so I think just because the region itself is based off of Britain it doesn't necessarily mean we're bound to British mythology too.
Here's an interesting theory I just saw: what if the Shield legendary has a rose theme? The reason for this being is that the Shield logo looks like it has thorns coming out of it, and that flower mural on the map looks like it has a shield on it.