I signed on with vayama.com through kayak.com (which still is sending me flight price updates daily, so I know whether I blew it booking for a July flight to Austria - or not!). Confirmation came within two days, so I am set on Lufthansa.
Other consolidators and booking websites (Expedia, et al) have nothing comparable in cost.
However, you do have another option - Virgin Air, which starts flying out of Seattle via San Fran (unfortunately, but check the special pricing!). You can then take Virgin to Britain. JetBlue also is offering special fares. You didn't mention when you plan to fly, but if it's a trip this summer I'd start setting fare alerts.