The journey time saving from HS2 is only part of the rationale.
As transtraxman states many lines are becoming saturated with increase rail usage, a situation destined to deteriorate further.
One of major issues with the current network is its multi-utilization having to mix very slow cargo with slow stopper local/cross-country with medium provincial and fast inter-city. Having such a variety does not allow for optimal utilization. Just like the performance of Ferrari or McLaren would be negated by sharing a part of a track with a saloon.
HS2 would remove HS traffic from the existing system allowing for far more use by slower traffic, especially cargo. Removing a swathe of cargo from road to rail would be a huge benefit both to the car user and the eco system.
So HS2 is far more than merely reducing times, it is about improving the rail network overall to increase usage and also reducing road cargo.
The issue I see is what type of HS – the current version or build for the future > 500kph ones currently being tested in Japan