Brit here, and a Welsh Brit at that. There's loads of terms that can be used for people from our islands, and some here, get confused too.
British, English, Welsh, Scottish or Scots (never Scotch), Irish, Manx.
As to the Idris Elba question of what he could be described as, I know that 'black' is generally accepted here, and is even listed that way in the police 'IC' codes for describing race ( https://en.wikipedia.org/wiki/IC_codes ). However, one term I've heard is 'Afro-Caribbean'. I've never heard it being used in a negative fashion, but maybe someone like @Nyotarules can tell me if it's offensive.
British, English, Welsh, Scottish or Scots (never Scotch), Irish, Manx.
As to the Idris Elba question of what he could be described as, I know that 'black' is generally accepted here, and is even listed that way in the police 'IC' codes for describing race ( https://en.wikipedia.org/wiki/IC_codes ). However, one term I've heard is 'Afro-Caribbean'. I've never heard it being used in a negative fashion, but maybe someone like @Nyotarules can tell me if it's offensive.