I think if they wanted to inform the debate they needed to have done so more than a decade ago, when lobby groups and individual ministers were first toying with the idea and related ideas. I think HS2, how it came to be the project it is today, how it keeps on going no matter what is a subject worthy of a national debate, as part of a general overhaul of democratic decision making. On a number of grounds I think it is less than desirable, so halt it and use it for the only thing I think it could be useful for which is analysing where things go so badly wrong in our society. I think we could get a lot more value out of the project that way. No matter how you argue for the project (capacity into London, capacity on the west coast main line, regenerating northern cities) I find there are always much more convincing arguments for doing something other than blowing £50 billion on a high speed train line, usually in a way that makes more coherent sense too.
£4 billion to each of Liverpool, Manchester, Sheffield, Leeds and Newcastle would transform their ability to deliver infrastructure, attract investment, attract companies, attract people... leading to less pressure on London, spreading population pressure to cities that need to regrow, leave the cities with money to spare, and total £20 billion (maximum, rather than £50 billion plus inevitable overspend). Were these cities more successful as entities in their own right, then you might expect London to account for less and less of their long distance travel needs as people increasingly connect between them instead, but on top of this why not invest in internet infrastructure and innovations that removes most need for such travel, reducing it simply to want?