I'm also looking at September dates to the West Coast.

Not using Avios though - availability isn't all that great. At the moment, the best fares I can find are from Oslo to Los Angeles for ~£1000 each return. You might want to check the forum dashboard here and read about ex-EU trips.

I second the earlier suggestion to look at the new flight to SJC (San Jose, California) for access to Yosemite - smaller airport, avoids the traffic around San Francisco airport and you'll fly on the new 787-9.

If you can get on the San Diego - Heathrow flight on the way back in Club, try to book seats on the top deck. Nothing quite like the big plane, small cabin feel!