Well oh boy, I'm writing my thesis on something similiar, which I should be worrying about but I'll bite.
As far as I can read and searching through Brill's New Pauly, the goddess Britannia wasn't "made" for the Romano-British in the way you might think it, but is rather a very complex and dizzyingly confusing subject of interpratio romana - whereupon local gods and goddesses are viewed through the lens of the romans (mostly their soldiery and merchants who arrive) and 'rebranded' into the roman psyche in a way.
Britannia seems to be a version of the already roman Goddess Minerva, an oft used interpratio of local customs and deities (another example was Sul, the celtic godess of springs and healing was simply called Sul Minerva)
There are very, very many examples of these sort of interpretations being made and it has, in my observation, been also a source of some debate on what was a wholly local diety, what was an amalgamation of several and what was entirely roman and so on.
Britannia was the name given to the entire isle by the romans and therefore Minerva Britannia would likely just be a way to register a local variant of her as they saw it.
Religious syncretism was big in the Roman Empire but the Romans didn't really bring gods from other provinces into their own pantheon, merely acknowledging other gods as 'the others' or bringing them into the fold by revisioning them as variants of themselves. Augustus is famed for having conquered Egypt and then brought back two obelisks which he erected in the Field of Mars and Circus Maximus and dedicated them to the roman god Sol. At the same stroke, he pushed the eastern gods (to which many had started to gain a foothold) outside the pomerium of Rome, which was the religious boundary of a city. Effectively banishing them to the countryside.
Notable exception, which incidentally my thesis is about, is the god Sol Invictus Elagabal. Elagabal was a syrian god whose origin is of some dispute, though some would argue it's a combination of the greek Helios, latin Sol and aremic Shams -- all solar gods. Elagabal was worshipped in Emesa (now Homs) and brought into the Rome in a rather spectacular fashion by Heliogabalus - a sixteen year old high priest of Elagabal from Emesa who was a distant cousin of the Severan emperor Caracalla who had recently been murdered.
Heliogabalus replaced Jupiter's spot on the top with Elagabal and promoted him as Sol Invictus Elagabal, the unconquered sun Elagabal. This didn't quite take and he was murdered by his Praetorian Guard three years afterwards.
You have other examples like Jupiter Dolichenus which was most likely a combination of the semitic gods Hadad, Ba'al and Teshub - just given roman flavour to it.
I'm sorry if this wasn't quite what you were looking for. To answer elsewise, I'm not aware of any "Mars Germanica" or "Jupiter Hispania" - and as far as I could read the "Britannia" was a rather obscure endonym for Minerva that doesn't seem to used before as a symbol and representation of a country much later, long after the romans were gone.
Sources:
Halsberghe, Gaston H. (1972), The Cult of Sol Invictus Leiden: Brill
Ball, Warwick. (2016) Rome In the East: The Transformation of an Empire New York: Routledge
Hijmans, Steven, (1996), The Sun Which did not Rise in the East