I don't mean to state the obvious, but indeed, if it's only Virgin customers, and then, only the PSN that's causing problems, then isn't it at least possible that Virgin throttle back the bandwidth for the PSN?
I remember years ago reading about BT Internet throttling certain ports and P2P services. Maybe Virgin have something in place to restrict bandwidth on PSN downloads?
I wouldn't imagine they would do this?
In any case, could also be worth writing to Virgin about, and asking?
From my own perspective (on Demon, not Virgin) it generally runs at least at a few (3-4) hundred k per second* (I only have a 7mbit connection anyway) though it can slow down, and start to time out, when we're all going mad trying to get the latest demo, all at once
V1N.
* I've never actually measured this, just watched the numbers going up the screen when checking in the download manager.
EDIT: Or has anybody tried using the PS3 to download a large file from another site, via the PS3 browser? If you downloaded the service pack or something, using the PS3 browser (you can do downloads, but only to a USB device/memory card I think) Just to rule out whether all internet access to the PS3 is restricted, or whether it is just the PSN that's running slow?
I will try and download a file onto a memory stick using the ps3 browser and i will post the results.

