the truth is, as you said, british are a fairly mixed people. the original inhabitants were the celtic britons, described by the romans as some of the most savage and primitive among the celts and proud producers of the warrior queen boudicca.
then the romans invaded and took over southwestern parts of england. although in modern times their genetic influence is basically null, it shouldnt be discounted, you still see some british with a light roman appearance to them.
then came the anglo saxons. if you are talking about "english asatru" then this is what you are looking for, the anglo saxons actually had a very complicated religious system and were in many ways some of the most advanced of the germans, it is sad that not much of this information was preserved but what we do know is very interesting. They had begun adopting many attributes of the Romans and obviously showed that the Germans were learning how to counter their enemies. While this mostly meant adoption of roman style armor and weapons (albeit with a heavily germanic flavor) sometimes this did mean a blending of roman and german pagan elements, with a heavy emphasis on germanic (this is why in english, we have "saturday" as an odd roman god's name among the other days, which are all named after anglo saxon native gods)
And then the vikings came and finally the normans, but honestly the normans are just french-speaking vikings. they radically changed the culture of england, but not really the genetics.
As far as which of these racial groups you actually come from, thats pretty much impossible to determine nowadays, at least from my knowledge. I myself am almost half english so I understand this struggle, I identify heavily with both the celtic and germanic sides of my ancestry despite their past quarrels.
The truth is, you are probably a mixture of several of these groups, for sure at least British and Anglo Saxon. The Roman, Viking and Norman genetics however are less likely, but not impossible.
https://en.wikipedia.org/wiki/Anglo-Saxon_paganism
https://en.wikipedia.org/wiki/Celtic_Britons
Hope this helps