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.