The reason virgin can achieve such high speeds is because they can target certain Areas as they so choose. targeting areas of heigh return on investment only. BT has to cover basically the whole of the UK. Theirin lies the issue. The chief bean counters need to keep it sustainable.
For example my modem connects to my cabinet at nearly <100mbps but I'm capped at 80. I live a stone throw away from the cabinet and less than a mile from the exchange.
The chief bean counters can't just increase your speed as the network is big. If you want to be proactive about it and get faster speeds you could get talking to neighbours who want faster speeds or to join and get local government involved to contact BT. If demand is there they could consider it.
That is just a tiny scale so you can imagine how much work goes into getting things like this done.