Cheapest Possible Flights to London

You watch them all. There is no one site with cheap fare because if it existed everyone would be there. Many of the sites have a fare alert or tracker option where you can put in a price and if the fare goes below that price they will notify you. I have never been able to get it to work but other claim success. Most of my fare alerts came in around 2 AM and when I logged on at 6 the fare was gone. And do not set an unrealistic price or expectation. For this summer from Mpls I doubt if it would be much under 11, 1200 dollars,

First thing is to look at other close gateways other than London; it's usually cheaper and more cost effective to fly into Manchester, Birmingham, dublin, or even Brussels and then hop on Air Berlin or Easyjet, or if in the UK, the train into London. I find that doing that, I can easily save $$ rather than flying straight into one of the London airports. I have even done CDG in the past with cheap results!

what ever you do, don't use airfare dot com. Complete SCAM.
Orbitz, Kayak and Travelocity are all fine. Not sure about priceline only because I am picky about airlines and depart times and the price bid doesn't give much flexibility.

Since I have been living in England, I have been using www.skyscanner.net, and they have been consistently showing the lowest fares. I also had good luck with fare alerts from kayak.com.