Negotiating terminals at LHR and a BA upgrade question

Hello
I'll be non-revving on a ZED pass from LHR to JFK or similar US airport in Feb. Can anyone tell me if most US carriers fly out of one terminal at LHR or might I face a mad scramble among terminals to if flights are full.
Also, does BA still upgrade non-revs to biz class if there is room? I flew BA from LHR to EWR about 7 years ago and was very pleased by the automatic upgrade. Since BA passes are electronic, our pass bureau said other ZED carriers don't accept them, but the chance of an upgrade certainly makes it worth buying a BA pass and getting other ZED passes as a backup.

You will find flights to JFK and EWR from three different terminals at LHR - 5,4 and 3 and the bad news is it is a pain to get landside from one to the other. Best way is to take the Heathrow Express train which is free between terminals, but only runs every 15 mins, so if you just miss one it is a pain.
Since you last flew on BA they have changed the employee travel rules for their own employees all of whom now have the chance to buy business class passes...which everyone does because it gives higher priority. So as an other airline employee you will only be upgraded if economy is full of revenue and there is empty space further forward. It will not happen as a matter of course or courtesy.
Best to check the loads on idDeals.com or similar and make a decision on the day which Terminal to go for. BA is the only NYC carrier from T5.
Good luck.