BT will eventually upgrade the exchange to 21CN - they are totally phasing out 20CN.
I find it suprising that 8 meg isn't enough for you. The uses you have said (spotify, youtube and xbox live) really shouldn't add up to more than about 2meg/sec if that.
Try getting a decent router with QoS and prioritise TCP ACK packets. It's likely that the 448k upstream is causing the problems, not the downstream.
Alternatively get an office connection and get the higher 832 (or whatever it is) upstream. Will be a few more quid a month but way cheaper than upgrading a phone exchange yourself!
Finally, if all else fails, get another line put in and get a load balancing router. You'll probably get another 8meg, so 16meg in total. Should be more than adequate.