Look for the RSPCA 'freedom food' approval, or the British tractor symbol.
Oh, and the EEC coding oval is a pretty good guideline, as it will only show 'UK' if the meat was slaughtered in the UK. I don't believe there's a massive market in importing cheap, live, foreign animals into the UK just for slaughtering purposes - but I could be wrong, and some of the less ethical retailers may well be doing this.