£1 out of every £200 that the government spends - assuming that is correct when one takes account of inflation - is a huge amount. One a single railway line. In no way is that a drop in the ocean.
If you closed 0.5% of all the hospitals in the UK, that would be 63. If you closed 0.5% of all the schools that would be around 1,500. Etc, you get the picture.
This thread is not the place to debate if Modern Monetary Theory works (spoiler: it doesn't) or if the silly trope about "GoVeRnMeNt SpEnDiNg Is NoT LiKe A HoUsEhOlD BuDgEt" means they can spend what they want (spoiler: Liz Truss had a go at that - it didn't work out well for her or Kwasi Kwarteng).
In terms of politics, it would be an order of magnitude easier (which means an order of magnitude more likely) for an incoming Labour government to cancel it and blame it on the "incompetent Tories".
Labour would get a lot more electoral bang for their buck by using a bit of money to paint buses or trains in the colour of the local authority and saying they've "taken it under public control" and funding some additional trains on the Transpennine Route Upgrade than by continuing to fund HS2. Indeed the leader of the opposition himself personally campaigned against HS2 works in Camden as it affects his constituency.
0.5% of schools being 1,500 would imply that there's 300,000 schools (there's about 32,000, so the number would be 160).
Likewise 63 hospitals would imply 12,600 hospitals (there's about 1,260, so the number would be about 6).
Whilst £1 in every £200 may sound a lot it really isn't that big a deal. To put 0.5% in terms of someone on £30,000 after tax (so about £38,200 total pay) that's £150 (or £12.50 a month).
For some, depending on their priorities, that's a significant amount. However if that's money going to a pension (in that some/all/more than invested is likely to be coming back to the individual, in the same way that HS2 will have at least some of the money coming back to them) that's different factor to if it's spending on chocolate.
Of course the other uncosted element is that of carbon emissions, in that rail has much lower carbon emissions than other modes of travel (yes HS2 isn't, under current travel predictors, likely to result in zero carbon emissions with various low single digets of millions of tonnes of CO2 being banded about, however that the total net value and should be compared to other factors, like Drax producing about 20 million tonnes of CO2 per year).
Arguably there should be a policy to build HS2 (and further rail enhancements to further reduce rail journey times) to then allow those internal flights which can be achieved within 3 hours by rail to be cut entirely and anything between 3 and 4 hours being taxed so that people try and limit their use of it. That would likely reduce further than the current model allows for, and so could reduce other costs (such as the reduced need for carbon capture, which has recently been given £20bn of government spending).
Whilst government spending on infrastructure isn't like household budgets, this isn't the case for government spending on costs which are ongoing. For example if I was borrowing £150 a year to buy chocolate it's perfectly reasonable to tell me that's a stupid thing to do. However if I were to be spending £150 a year for 5 years to insulate my house, that's a less stupid thing to do.
Even if they spending on insulation had no certainty that I would recover all my costs over a 15 year period. However, even if the insulation has no resale value after 15 years, it's still likely to be of benefit (even if that's a less draughty house).