Am I British?

Hi guys! I just wanted to ask this question to anyone who is what you'd call "real British" (i.e., has been on the island for generations). I'm originally from eastern Europe, but I came here when I were about 10 (year 5). Although not having knowledge of any English before that, I quickly learned the language to the point that I now have a very heavy Northern (Lancashire) accent. I have adapted very well to British culture, and I would definitely say I see myself as a Brit (more Northerner than anything else ofc hehe). But some people seem to have a problem with that when I say this. They say things like "oh but you're not actually from here, you can't relate to us" or "you'll never be British", which really upsets me sometimes because I know Britain more than I know any other place! I made my first proper friends here, I grew up in the culture, and I deeply connect with the country on a personal level. So is it acceptable for me to call myself British?