Okay, so I wanted to work this out for myself and check definitions. All of these are reductive and could be disputed but...
Citizenship is legally/politically defined and legislated for and applied or granted by soveriegn states. Rules vary accross time and place.
Nationality is tricky and I am struggling to define it - it has some legal definitions, but is also about birth place and allegiance to that birth place? I don't know how Reform or the far right define it, or if they bother to. Something about allegiance to the place you are born? This keeps coming up "While all citizens are nationals, not all nationals are citizens—only citizens typically have the unconditional right of abode". Which is interesting and if legally true means Nationalists are thinking about Nationalism all wrong! But I digress...
Ethnicity is a combination of culture, geography, heritage/ancestry and religion, often self defined, but recognisable? Sikh, Romany, Jewish, Irish, Welsh, Ughurs, Suni, Kurdish, Persian (all combinations of place, religions, relatives and culture to a greater or lesser degree, many subsets and groups within the broader terms).
Race is some made-up hierachical shit mainly based on the colour of skin (ie not white) but pretends not to be about that. White, Black, Asian, Aryan makes no logical or scientifc sense and only serves to reinforcing perceived difference for the purpose of power. Imposed on people by others
So,
You can be a British citizen with English ethnicity and racially white. And Nationally .....British.
You can also be a Chinese Citizen (i don't really know the Chinese rules) with English Ethnicity and white racially. And Nationally British?
You can be a British National, a Spanish citizen, ethnically Sikh and racially Asian
And then it changes by country, law, date, the commonwelath and politics. So much of it is just the luck of the draw too. Argh - I thought I was getting clearer but am back in the mud!
I think there isn't a British ethnicity, but if there is definitiely Irish and Welsh, then English and Scottish count. A combination of culture, religion, heritage and place. They are not as homogenous or tightly defined as once they might have been, or possibly never were or have infinite variations within them.
Almost all of these words and ideas are used interchangably and pretty randomly, and are often misued and misunderstood, but they do have meaning and significance. Stuart Hall called race, ethnicity and nationality the "Fateful Triangle".
Sorry that was long. No one will read that, but useful to me.