On google, the definition of racism contains, ”antagonistic towards people on the basis of their membership of a particular racial or ethnic group”.

Britain contains a large ethnic group of White British. This ethnic group is what op is referring to when talking about how they colonized countries. British is also a nationality, in which being against a nationality is, although not a race, still a form of racism. There may be a better word that I do not know of. I think it still can be offensive to be against a certain nationality though, even if ’racism’ isn't the right word to use. It’s just probably not best to make statements relating to nationality on platforms like Quotev.