If you want UA metal, then it's FRA but that's going to gain you only an hour or so. If you are willing to go AC, then the 7pm from LHR is the way to go. Remember that you gain an hour from GVA to LHR so you win that way. Where you lose is that the only airline flying GVA to LHR is BA, so that leg cannot be *A. Also, their 3:10 flight, arriving 3:55 is the latest you can take. The one after gets in at 6:10.

You might just have to explore BA, VS or AA, all of which have late flights out of LHR. Specifically 5:25 out of GVA, connecting to 8:00 out of LHR to JFK should fit your requirements, but it's BA.