thanks all for suggestions and especially, evelyn for help. i ended up doing one-stop in both directions on united. the price was so much lower than any conceivable combo with a delta nonstop from lhr-msp that it made little sense to wait and risk those prices rising too. we're going msp-ewr-edi and coming back lhr-ewr-msp. less than $900/head. will put the money saved into other parts of the trip.
the boys are campaigning for edinburgh to london by train but i think we might do a cheap flight instead. as our first few days in london will be spent with friends in kingston, i'm assuming gatwick is the non-heathrow airport that i should be looking at. (do any of the cheapo airlines fly out of heathrow?)