OK, time for some damage control. Seems like there are a few misconceptions that need to be clarified.
First of all, there is a difference between route authority and a slot.
A route authority allows an airline to fly a specific route between 2 markets, such as New York City to London. (notice I did not mentionn specific airports.)
A slot is a specific right to land an airplane at a specific time at a specific airport.
Therefore with a route authority you can serve a market, but only to an airport with no slot controls (ie: Gatwick) or an airport with slot controls for which you own a slot.
What does this all mean for UA? Well once the route authority is sold, UA will no longer serve the NYC to London market. However it can use the LHR slot from any other city for which it has route authority. (Such as ORD or IAD)
While it is sad for JFK employees to see flights shifted elsewhere, it is smart for UA to move assets to where they will create the greatest revenue. Keep in mind that UA is leaving a market it was generally weak in , and expanding it's strengths in the Pacific, while at the same time building up the hubs in ORD, IAD, and SFO.
UA has not had a strong NY presence for some time now. Delta, with it's hub in NY will make better use of the route.
I do not see this move having any effect on PS service what-so-ever. UA has been clear since the start that the PS transcon service is a money maker. The feed to/from LHR was minimal and will be redirected to other hubs. The feed from NRT was nonexistent. (No one goes from NRT to LAX or SFO via JFK.)