In any case, if HS2 was truly about capacity rather than being a high speed vanity project it would have been specified as a 125mph (or maybe 140mph at a push) railway so that ordinary stock could run on it rather than having a much more expensive specification and requiring very expensive brand new stock to utilise it.
High Speed rolling stock isn't really that expensive!
A TGV Duplex only costs about €20m for a 200m trainset. (EDIT: More recent figures for the higher capacity TGV-M put it at €30m, but still not substantially more than a UK spec 200m conventional trainset).
You won't get a UK gauge conventional train for much less than that these days!
And has been repeatedly demonstrated, building railways for lower speeds doesn't really save any money!
You save a bit on the track specification, but that's pretty much it!
HS1 estimates put the saving at under 10% for 125mph vs 186mph.
And HS2 has to be fast or it won't be able to relieve enough main lines to actually use its huge capacity.
EDIT:
A lot of people are desperate to blame specification "gold plating" for this debacle.
But that's not what caused this, what caused this was bad management and an overreliance on consultants who's reward structures in no way align with the public interest.
As well as a fundamentally outdated view of how to build infrastructure.