You might want to check what times TK's check-in desks operate at LHR. They only have 3 flights a day, so it's possible that the desks won't be staffed some of the time. If the luggage is thru checked, then you can probably get the boarding pass at the ticket desk.

Also, re lounges at LHR T3, i've used most of them & whilst some are better than others, none are really dire. Thing is, the one I haven't used is the servisair one.....