Well, it was Aug last year, but I managed to check in at LHR T1 at 07:10 for my 21:00 flight to JNB in C - I make that about 14 hours before my flight. The person in check in did comment about how early I was. I explained that I had to do a business trip for the day to EDI before my holiday that evening. Got the bags checked in without any further problems and got my boarding pass.

I then walked from the T1 premium check-in to a self-check in machine at T1 domestic to pick-up 2 more boarding passes for my EDI return trip.