My guess is that the B787-10s will be on the LHR-JFK route and will be in a low Y configuration. The B787-10 can't fly very far when fully loaded.
There will probably be a second daily LGW-JFK high density B772 to supply more Y seats.

The A350s may initially be used on routes to allow the B773 aircraft to be converted to 3-4-3 in Economy and replace B747s as these are retired.