There may not be an economic case at the national level, but at the European level there's certainly a political dimension to HS2. It links the major UK cities into the european rail network. The whole project is built to a continental loading gauge and will be running continental freight as well as passenger services. This is because it's all part of the ongoing process of transport integration across the EU.
In that light there might be a better economic argument to be made than "it'll get us to London a bit faster". Better-integrated transportation links within the EU can be argued to have a very positive potential economic impact, though of course the primary drive of everything the EU does is "ever closer union"... perhaps that's why our politicians are so leery about giving credit where it's due.
And of course transportation policy is an EU exclusive competence anyway. HS2 would likely go ahead no matter what.
You may think this is a good thing, you may think this is a bad thing. What annoys me is that our alleged betters in Westminster feel the need to lie to us about the source of it all. They take credit for things they haven't done- oh wait, they're politicians, that's all they ever do anyway...
tl;dr an economic case can be made if you realise that HS2 has an EU dimension; it's the EU wot done it anyway. Why won't talk about either of these things?
Incidentally, it's worth noting that travelling first-class off-peak with Virgin is only £15 more expensive than standard class and you get free food and booze for that, plus access to the first-class lounge in Euston and all the free hot chocolate you can consume while you wait for your train to finally turn up after all the delays. No mortgage required.