"the only British name in Mrs Windsor's ancestry is Owain Tewdr."

Are you saying that names like Douglas, Mure and Drummond etc didn't originate in Britain? Likewise her tree goes back to pre-surname times anyway and her Pictish ancestors would be every bit as British as her Welsh ancestors. The Roman frontier was imposed artificially by outsiders. Or are you saying only people with P-Celtic names are British? Surely a jest. The ancestor of the Welsh language is itself not necessarily indiginous either :-)