The smaller tables adjacent to the bar at LHR T5's Plane Food seem reasonably comfy for a relaxing drink and nibbles. In LHR T3 I'd opt for a glass of chilled Chablis (and a seafood salad) at the Caviar House counter.
Otherwise it tends to be bars in lounges these days. But people watching at LHR can be quite interesting so sometimes opt for the venues mentioned above.