It doesn't matter how fast the server connection is, you won't go any faster than the slowest link between you and it (so even if it were 100M all the way to ntl, your connection is 10M, so you cannot go any faster than that).
If you are only getting 300KBytes/sec then it could be that the links from ntl to the server are slow (or just very busy) - is it in the US by any chance ? It could also be congestion on your local UBR (oh, and ntl's proxy servers are not always as fast as they could be either

).
BTW a 100Mbit connection is about 12M bytes/sec, not 120.