I go frequently to LIS and have been using the west coast SFO-LHR fares a bunch that allow for a stopover in Lisbon (with no min stay req. in London), comes out at about $2,350 per pax roundtrip instead of playing the GPU lottery. Tho it would have to be for July due to ticketing limitations.
Fare basis is: PLXM6SNC.
Main restrictions are:
ADVANCE RESERVATIONS/TICKETING: CONFIRMED RESERVATIONS FOR ALL SECTORS ARE REQUIRED AT LEAST 180 DAYS BEFORE DEPARTURE.
OUTBOUND - PERMITTED MON THROUGH WED ON EACH TRANSATLANTIC SECTOR.
INBOUND - PERMITTED TUE THROUGH THU ON EACH TRANSATLANTIC SECTOR.
STOPOVERS: 2 FREE STOPOVERS PERMITTED ON THE PRICING UNIT - 1 IN EACH DIRECTION IN LIS/OPO/REK CHILD/INFANT DISCOUNTS APPLY.