The lessons have not been learned from HS1. Tickets for "High Speed" services are around 25% higher than normal services, passenger numbers are lower than expected:
http://www.nce.co.uk/news/transport/high-speed-1-underwhelming-passenger-numbers-exposed-taxpayer-to-debt-risk/8628492.article
Its a no brainer, if you are a commuter, if you get up 20 mins earlier you save 25% off your commute cost. Other way around, why would you pay 25% more just to arrive 20 minutes earlier?
Only ones that would pay that are people wanting to "experience" high speed (low numbers) or business travels that can be expensed (again not great numbers)
The bulk of season paying commuters don't like what they are paying now, all you need is Chiltern/West Coast to be cheaper and HS2 is a dead duck.
HS2 money should be spend on current infrastructure...
But as I stated Tunnie, these high speed lines are more about the future than now.
If only Brunel's Broad Gauge had have won the day the railways would be even more advantageous than they are now 
So spend that money on the lines we have now, make them faster, better signalling, better faster rolling stock, Electrification of what we have.
Get what we have up to scratch first, then look at new lines.
Indeed Tunnie, that would be a good approach in places, and the rail authorities are spending money in that direction when they can

However, in many places it is cheaper to build a brand new fast link than use existing track beds. In our busy, over crowded cities, they have grown up and developed around the original Victorian railway infrastructure. Without tearing down buildings, extending track beds, building new tunnels and bridges, along with extended stations, you cannot just lay additional track on what exists.
Crossrail under London is a good example of what has to be done now; build completely new infrasture, in this example tunnels, to take the new track to create vast additional capacity.
The lines that exist currently all travel through major cities, I cannot think of one where it would be 'easy' and cost efficient to build in the additional capacity without major demolition, huge cost, and massive disruption. Again, it is far more cost effective to build a new high speed, for the purpose, line from scratch. Never underestimate also the difficulty in building into existing curves, straight but limited track beds, and as stated before, the railway stations themselves.
Here 'down South' the HS1 line was built in many cases within sight of existing lines, but there is no way it could share the old track bed and route in general. Indeed in Ashford the HS1 goes over the existing station via a motorway type flyover. Ashford is a fairly small town, so just imagine the problems with very large towns and cities.
No, it is either a brand new line or nothing. Like additional airport capacity, doing nothing is not an option for the commercial interests of our country.
