They're phasing out 20Cn - it's so last year. So there's no point in investing in it.
And there will be plenty of market 1 exchanges with 20cn for some years to come. No point in BT upgrading them to 24 meg when there's no competition.
Then it's time Ofcom rearranged the market 1,2 and 3 pricing structure so that users who can get faster 21CN speeds pay more for it than those of us stuck with 20CN snail speeds.
After all you pay more for a Royal Mail first class stamp than you do for 2nd class in the hope it will arrive at the right destination in a faster time
By the time some areas manage to get 2Meg syncs the rest of the country will be getting 200Mbps using second generation fibre optic combinations.