What BitTorrent client are you using?
Most allow, at least in advanced settings, these numbers to be altered.
1) Do a speed test, when not using the internet at all (apart from speed test):
http://www.speedtest.net
The upload number is important.
2) Check this site:
http://www.azureuswiki.com/index.php/Good_sett ings
Match your settings to roughly the settings required by that table (based on your upload rate, measured in 1)).
Basically you want to change your upload speed to match about 90% of your total upload speed (your app may measure in kB, not kb), and then also the number of max simultaneous connections).
But mainly, just because your download is quite large, your upload is still small. So, the connections are still going to be limited. (Doesn't have to be this small, but that's what most ISPs in the UK do...).
Also note that Virgin used to be a good ISP, but in the past few months seem to have started doing some more questionable things, to try and stop piracy, etc., at the expense of those using technologies like BitTorrent for legitimate purposes. This could also be a factor in your case, but that may be far more difficult to find out (and there's not much you can really do about it anyway if this is the case).