4 cheapest revenue seats to YVR is ~5300 in Y for the flights you indicate.

Three D seats (that's the code for the J award seat) and an ex-SEA Z fare gets four people to LHR in business class for about $4500. That's pretty good. Avoiding the trek to SEA costs you $1000 at least.

You can fly one way YVR-SEA for $125, or 11500 points + $50 or so, to meet your first flight. You end up having to go to the airport a few hours earlier than the other 3 people.

At the end of your holiday when you arrive in YVR and it is time to catch that last leg to SEA, well, obviously the luggage is all checked with the 3 other people and you have carry-on only, and who knows what personal emergency might crop up preventing you from taking that last flight to SEA you don't want to take anyway :hint: :hint:

Note, if a great deal in J crops up between now and then that beats the ex-SEA fare, you can cancel the Z fare for a $200 penalty.