I'd suggest either ask folks to pinpoint the one option which they most closely identify with or allow the option of multiple boxes - and have a link there explaining what each grouping means too. In the UK we are a real mixed bag racially speaking. I for one could go for the first three categories having a mixture of native Briton, Welsh, Irish, Manx and, looking far back, probably Norwegian Viking blood, and like most English people probably a bit of Saxon too, maybe even some Norman (that's only if one of my ancestors got lucky and married someone posh

). I suppose it depends how far back you want to go, if we all went really far back we'd all be the same wouldn't we?
I'm confusing things as ever....