Many thanks to
@Bald Rick for his invaluable posts (and his patience). If I may, I'd like to add a couple of words on funding as one who used to be involved in public spending.
First, there is always more demand for public spending than there is money available. This is not a political comment, but everyone can think of something that will be wonderful and cost more than their personal taxes can fund - so there is rarely a meaningful upper limit.
Second, it would theoretically be possible to run a BCR over every piece of government spending and fund things on this basis - highest BCR first, lowest last and fund projects down the list until you run out of cash. But even if you could agree a methodology (you won't) then the results would probably be (i) primary and technical education (ii) pure science and engineering research (iii) infrastructure and (iv) healthcare for those below retirement. Health is an excellent example of a skewed 80/20 rule -half, and perhaps as much as 75% of medical spending is in someone's last year of life, which is clearly terrible value for money. So, we can save lots of cash by closing hospitals and not committing money to the elderly, and maxmise BCR in transport by only investing in projects around major metro areas, and especially in the SE.
This is clearlt politically impossible, so what actually inevitably happens is that everyone gets something - and yes, this means that some rail projects with higher BCRs will go unbuilt whilst others with lower BCRs get the nod. Look closely at the winners from the 2010/11 transport elements of the spending review - and what has survived: WRATH (T May, Maidenhead; D Cameron, Witney); Electric Spine / MML electrification to Sheffield (N Clegg, Sheffield Hallam). So even a technocratic answer is inherently political.
Third, though the UK remains one of the most fiscally centric nations on earth - most cash goes into the Treasury and then comes out again, rather than being raised locally or regionally - which is a strength and a weakness. What it could, but doesn't always/often mean is that strategic decisionmaking is easier. Instead (and MoD is probably the worst offender here) what matters is process, and getting your project into the system. If you can get a project accepted, then it is increasingly difficult to stop, irrespective of cost overruns. This means that all of the incentives are in place to bid unrealisitically low and stretch everything out in terms of time and cost - perversly it is in some sense to DfT/Grayling's credit that they pulled MML wiring when it was clear that it simply wasn't deliverable inside their fiscal envelope (and Clegg no longer being Deputy PM made the politics much easier).
Fourth, and for now, finally (if indeed anyone has read this far), capital investment is always easier to cut than staff, even it is critical over the long term. This is why Brown as Chancellor set up split Resource / Capital budgets, so that departments couldn't just raid the capital budget to pay for staff or programmes. Crucially, departments could move as much cash as they wanted the other way (more or less, it was never really tested) to allow more capital investment- because the easy way would simply result in much lower capital investment.
All of which is saying that "political support" or "funding in place" may mean a large pile of cash to hire some bulldozers and buy some track, but more likely is means that there is "broad support" to one (or more) of the stages that Bald Rick has so helpfully outlined above.