Unlike Vgaslady whose one concern is geting to/from London, you have the added complicaton of wanting to meet up with friends. I have to say that usually when I've taken these ( I'll non-rev and meet you there) trips things have worked out well. However, there have been the few times (and trust me, friends will never let you forget those) that I didn't show up be---cause I was struck in Dallas (now I ask you, could I help it if they closed the airport because of thunderstorms and wind gusts of 60 MPH, I was willing to fly.)
But, because you have 10 days and are flexiable, Im going to come down on the side of non-reving, with the caveat that you get plenty of back-ups on other carriers, plan on arrving late and leaving early, invest in a good lucky charm and don't come looking for me if London turns into the springbreak place to be. .

Edited to add vital stuff
You might also want to read Al's How to non-rev article, lots of good info.