Re: connectin flight...
I have been through DFW many times and its simply huge. As you are arriving into DFW on an internal flight you will need to
transfer to the international terminal which is at the other side of the airport.
I would very much doubt that you will make the connection in 40 mins.
Did you book the flight or did a travel agent arrange for you?, if its the latter then you should take this up with them because its there job
to arrange connections with plenty of time, even accounting for short delays for inbounds.
Having looked at your flights, I would guess that your going via DFW because you are flying with AA. If you are and your arriving into DFW with AA and you miss your flight then they will have to re-book you.
If your arrving into DFW with another carrier and then leaving DFW with AA and you miss your flight then it will be up to you to make arrangements. to get to London
I hope this helps you and I wish you luck with your flights.