I am experiencing this issue too. If uploading at even 8KByte/s (out of max 60+) then downstream suffers immensely.. About 150 - 300Kbyte/s downstream!! Now my connection has always been pretty ****e, with fluctuating pings (Can't believe a telco haven't been able to sort it out in a few years.. but hey). Downstream is near the full 10MBit/s whilst not uploading.
Even on 3mbit I could upload at 20 comfortably.
BBKing, if you're still around are you able to check congestion on my UBR, if i PM you my MAC?
P.S. Forgot to mention, start getting about 25% PL to NTL's routers when uploading at about 20KB!