I would hazard a very tenuous guess that it's not as simple as UK owing US x and US owing UK y -- some of that may be owed to private corporations, individual banks, etc. That, and having a lot of debt going both ways is possibly something of a security blanket.
Ah, so in this case, it's not necessarily the Chinese government, for example, that owns US debt, just companies based in China? That makes sense. In regards to the security blanket, this would mean that if the UK failed and couldn't pay the US, then America would just say "That's okay, we owe you tons of money anyway?"
Keep in mind that a lot of Chinese companies are government sponsored (owned), and many may have CEO's that were previously party members.
To the first point, I have no idea if that's actually the answer! Normally I would read Wikipedia until I knew one way or the other but tonight I'm trying to read other stuff. It seems logical, though. Second point, I just see it more as a mutual affirmation of continuing business partnership, or something. There are other reasons that are nagging at me. I'll read up on this sometime.