Everybody that isn't BA/AA is 'extremely weak' on the route. All the other airlines in the NYC-LHR market combined equal about 60% of the capacity of the oneworld JV.

With that said, I don't think it's a given that a F cabin on NYC-LHR is a must-have. I have heard that many of the pax in BA F cabins are connecting onward to Africa and the Middle East, and that BA has taken a hit there with the entrance of Gulf carriers to the nonstop JFK market. The business class cabins are critical for NYC-LHR terminating pax, many of whom are under corporate contracts (and BA/AA have a great deal of those).

UA cannot reasonably serve the NYC-LHR-CPT F passenger, and would prefer to send the NYC-LHR-DXB F traveler via IAD. That's why I'm not sold that EWR-LHR must be an exclusively 3-cabin product as a competitive necessity.

On the other hand, there is probably a stronger business case for 3-class service on EWR-FRA, with ATI and revenue sharing with LH.