If this is true just raise the fares for business travel and HS2 will have a financial case. No BCR required.
All that would do is make people opt to drive or fly.
If we take London to Birmingham is about an hour faster by train for Euston to New Street travel, so if that's what you're doing then it's a no brainer to do so, even at £106 return (which is the same as 118 miles each way at £0.45/mile).
However if it's travel between Croydon and Jewelry Quarter then the journey time is the same and so the ticket price starts to get more sensitive, as does the frequency of services at each change point.
However if the to is between Guildford and Jewelry Quarter then because the train takes longer (about 15 to 30 minutes longer each way) than driving; then the ticket price will need to be quite a bit cheaper than the £122 to encourage rail travel over going by car.
Given that you can't give people their own ticket price, so that longer and slower journeys are cheaper than shorter quicker journeys (especially if the longer route uses the shorter route), then how do you set prices so that you get the most income?
Chances are you set them cheap if you book in advance and then increase the cost the less notice there is for travel and the busier the service is.
Now whilst some business will get some good deals there's going to be a lot which need to book last minute or even just buy a ticket having just been told to go.
Overall it broadly balances out. However if the tickets are too high few will use the service.