Can UK Muslims eat non-halal meat, such as McDonald's, on the basis that Christians are people of the book?

So yes Christians are people of the book. However, we don't know whether the butcher that slaughtered the animals was Christian or Jewish or Muslim or whether they were atheists and had no faith, because here in the UK there are a large proportion of people that are atheists. So because we don't know, it's not permissible.