HS2 is probably going to be £300+ for a peak return from London to Leeds - yes, I pulled that from my backside, but given when it’s due to launch and the ever-ratcheting fares, I think it’s perfectly realistic.

As to supply and demand influencing pricing - yes and no. While there will be more capacity, existing operators and franchises may or may not take advantage of it, and even if they do, what possible motivation would they have for reducing their prices? They won’t be competing with HS2 on price - if anything, the inevitability high fares for HS2 will provide an anchor for a price increase on the old mainline. The only others trains running on the same route will be theirs, as that’s how the rail franchise system works, so their only competition will be themselves - i.e. none.

The problem at the moment in off-peak times is that they dare not reduce the price because there are trains that run during the day which fill up, and there's no good way of differentiating fares between the ones which do and the ones which don't.

Isn't there? Travelling from Leeds to London at 7-9am Monday costs several times as much as at 2pm Sunday, and I assume that's because the operators can differentiate which journeys are popular (and furthermore which ones carry large numbers of business people who can afford/expense higher fares).

imho, to prepare for the long-term we should be building infrastructure on the ground to reduce short haul flights.