One of the possible problems here is that the throttling may be taken from the Virgin media M 10MB package which may well give a speed "up-to 10MB" and not the 10MB L package which has a different throttling limit.
Another small possibility is, at one time I was told that the throttle limit actually "combines" our upload and download activity, so if your (M package) so as soon as your "combined upload and download has reached it's limit you get throttled.
(Although I have deliberately tested this theory on several occasions and it does not seem to be true)
See the following table for UL/DL limits.
http://allyours.virginmedia.com/html/internet/traffic.html