The scenario of 20-30 F + GS passengers ahead of you seems extremely unlikely. I believe all three ORD-LHR flights are 3-class 767, with 6 F (1 blocked off) and 26 C seats. The chances of 5 F passengers and another 15-25 GS seems remote at best.
So, assuming they did this by status, another question becomes why are they giving 1K's on a cancelled flight seats taken from a full-fare passenger on a following flight? I would have been very upset too.
Also, and more importantly, why isn't UA making an effort to re-book those disserviced passengers, GS or otherwise, on another airline? Really poor form on UA's part.
We all hate to experience cancelled flights, but even as a 1K I realize that its going to happen to me on occasion. I don't expect them to be off-loading full-boat passengers to accommodate me, but I sure expect them to re-book me as necessary to get me to my final destination.
It sounds like the whole situation was very poorly handled, but I'm glad you finally got to sit in the C seat!