Currently it is very strict and they won't let an extra inch (or 20%) excess go. But if you are in transit they are more lenient, particularly if you are eligible for FastTrack (flying BA F/J on your connection out of LHR), and I think you would be OK at the FastTrack check. The intent is to prevent the rollaboards and not so much thick briefcases (though 6 inch is pretty generous, I have a big case and it is only 5 inches, and my "nice" briefcase is actually under 2 inches).