In LHR, I like Perfectionists' Cafe (T2), Rhubarb (T3), Carluccios (T4 airside/T5 landside), Plane Food (T5), and Tea 5 (T5 Sofitel).
Worth mentioning that it's allowed (although not exactly encouraged) to change terminals airside to visit a particular restaurant at LHR, so an incentive to spice things up on those long connections