To follow up on tom911's approach. Another routing is to fly BLI-SEA-LAX-LHR. The SEA-LHR leg is worth 6410 miles and the BLI-SEA run will get you another 500 miles. (If one wanted to live on the edge you could try to change the SEA-LAX leg to SEA-PDX-LAX to get a few hundred more miles.) An advantage from Seattle of going down to LAX is that SEA-LAX is not a shuttle flight. The transition from C on LHR-SFO to Y in SFO-SEA is rather jarring.