Most genealogists treat ethnic origins as a bit of fun and not to be taken seriously. We use it to back up what we already know from research and to help break down brick walls (If we share DNA with someone, we know we are related via an as yet unknown ancestor). There is not enough data in the system yet to be accurate. My ethnic origin has changed several times since I took my DNA test - with the same tester.
I'm currently 61% England & north west Europe; 14% Scotland; 11% Wales; 5% Ireland; 5% European Jewish; and 4% Sweden.
They removed the small % of Africa a few revisions ago.
Sweden I can understand - most British people have some Viking blood. North west Europe - Normans, Saxons etc. I have no southern Europe though (yet) - maybe my Roman ancestors' DNA has been diluted out by the more recent invaders.
I Know that virtually all my ancestors for the last few hundred years are English by birth - though that doesn't mean that further back they didn't come from somewhere else. I know that a great great grandmother was half Welsh. But I have several illegitimate ancestors and fathers of these children were not recorded (though I have discovered a couple from the Lancashire Quarter Sessions where the father was ordered to pay the parish for the upkeep of the children); their fathers could have come from anywhere and with current knowledge there is no way to find out who they were, let alone where they were from.