I've noticed large variations in download speeds with Virgin cable. some programmes come down at ~ 40 Mbit/s others at ~ 10 Mbits/s
Looking into this I see that the fast ones come from somewhere like m65-mp2.cvx1-a.lis.dial.ntli..net which appears to be a virginmedia site in Luton.
The slower ones come from ae-2.r00.lsanca07.us.bb.gin.ntt.net which seems to be somewhere in California. 
I'm guessing that Virgin are caching iPlayer content, but why in California?

I'd be interested in looking at this, but forgive my ignorance - can you layout how you found these two addresses please?

At first look I'd say those addresses were the last hops before you were hitting the CDN that serves the iPlayer streams so potentially it's not virgin caching but perhaps some kind of routing being done by the CDN.

But if you tell me what you did to identify these addresses I can take a look (I'm with Virgin).

I found the addresses by watching the Network Activity for perl.exe in Windows Resource Monitor during a download. It seems to be quite consistent.

Square Penguin. Did you try looking at the Resource Monitor? There is absolutely  a huge difference in download speeds depending on the network address shown in there. Also seems to be a difference in the sites used depending on where you are on the Virgin network, I am in Coventry, but was recently in Croydon, also on a Virgin connection and it looked very different.

I believe Virgin Media have been caching iPlayer sources since forever. I think it was an invisible proxy in the early days, and I remember that could be a bit flaky at times. I'm not sure how they are doing it now. Where I am in the North East, the download speed hits my network limit most of the time, so they come down full speed. At least it's less load on the BBC, but I wonder if it counts towards their viewing figures?