You can't tell if someone is "native British" by looking at them. There's records of non-white British citizens with voting rights back to 1749. Are those people's great-great * whatever grandchildren not British if every generation has lived in Britain because you don't think they "look British"?

Britain is not and never has been a single ethnic state. It's very hard to not consider someone a racist who thinks so.

Tommy Robinson, as supported in this post, was a member of the BNP. The BNP, until 1997, had a policy of forcibly removing all non-white people (including citizens) from the UK. After 1997 (and when Robinson was a member) they moved to a policy of "financially encouraging" non-white people to leave the UK.

If you are a member of a party who believes in treating citizens differently based on the colour of their skin: you are a racist.

If you think you can identify British citizenship based on the colour of a person's skin: you are a racist.

I don't really like throwing the "racist" word around but this is literally as simple as the definition gets.