I used to always catch NZ6 and then NZ2 to LHR just for the In-N-Out transit. IMHO the ~25 min walk each way and a great burger was the ideal way to break up the journey.

It's worth remembering on the way back that you can consider the *G fast track (assuming you are *G) in the United terminal and then the airside connector to walk back to TBIT which can be a lot more enjoyable than TBIT security. Just allow some extra time if you've never used the connector before.