I've always found that award space on SEA-LHR has virtually never been availalble (including dates 6 to 10 months out), except within perhaps three weeks of travel on flights which are less full. Almost every weekend, the dates are shaded, where SFO, LAX, and YVR always have space.

As an alternative, I've always been able to find space between YVR and LHR so when I book award travel, I always have them book SEA-YVR-LHR-YVR-SEA. (Internet will never pull this up so you'll have to call and specify this routing to obtain this.) There is no additional charge for Alaska Airlines flight segment provided award space is available on Alaska flight. You may have to pay the rez agent fee however.

Jiburi