payingpax - Who are you flying with?
If you're flying BA, check in online. If you're a member of the Executive Club and you've got a PIN, you can do this from 24 hours before departure. If you're not, you can just register at the BA site and check in from 12 hours before departure. You get an earlier crack at the seats, too, although this won't always get you the best.
When you've done that, your (absolute) deadline for arriving at LHR and picking up your boarding pass is (I think) -50 minutes (or -60 minutes at Paddington). You can do it at a self-service kiosk or at the "Fast Bag Drop" desk where there is seldom any queue. Saves an awful lot of hassle if you don't have business class check in privileges.