Alrighty, my mistake. In the setup of the mirror infrastructure, I stuffed up the server alias logic so that some yum and apt-get requests will be failing at the moment (specifically on the us mirrors). The uk mirror should be ok. Either temporarily override your config for the uk mirror, or wait a few hours as things gets sorted on those systems. Sorry!
Update: all fixed now – and it seems the mirror people are smarter than I anyway
We’ll get some testing and monitoring in place for this kind of thing, it’s obviously very tricky to spot such problems otherwise.