Which part of Plymouth are you in? I'm in Beacon Park and I'm on the 10Mbit package too and I've just run a speedtest and it only hit 4.3Mbit all 3 times that I ran it. Could possibly be a problem with some equipment down here, although I'm on ubr08.newt.blueyonder.co.uk.
By the way, I've NEVER managed to get a full traceroute to telewest.co.uk or blueyonder.co.uk in my life, so you're not the only one