Traditionally, JFK has always been the easiest route to pick up reward seats on. This is purely because of the amount of seats available.
There have been no complaints on V-Flyer about scarcity of UC reward seats on the LHR routes. The problem is on the LGW routes to LAS and the Caribbean, because there are only 14 seats in UC on those planes and they are very popular.
Nick
Edit for typo