It's strange to go through Canada from DFW to LHR when you can probably go non-stop or at least change in the US.
According to Expedia, it would be cheaper for me to connect in Toronto. Booking a nonstop flight from DFW to LHR would be $1,000 USD more.

And I couldn't find an itinerary that included a connection within the US- at least for the date that I needed to fly out.