Could this be an isp issue or is it likely my modem even though it syncs the upstream at 2123Kbps?
If this was your downstream, it would be an obvious profile issue. Since its the upstream, I think you should ask your ISP (which ISP are you with ??)
Annex M over BT's WBC network is fairly new; and given the amount charged, I don't think many people have experience of it yet.
pro - on THFB - sync about 17.2mbps - BQM