'Celtic' will include some Scottish music, e.g., but the purist will add "from the Celtic areas", i.e. Gaelic-speaking. Originally, that is. The rest is from the Lowlands, which (speech-wise) is on the same continuum as Northumbrian, shall we say. As said above, the music of the North of England is on a par with "Celtic", and is actually a very good example to pick. Your Southern music, from the home counties, Zummerzet etc., may be typified by morris stuff, which may be wimpy compared to loud pseudoAmerican rock, but is for me typical English [non-Celtic] music. If Celtic is what Enya et al. sing, then English is what Maddy Prior sings.