Maybe an interesting point of comparison:
The Swiss NEAT-project (mainly the Lötschberg, Gotthard and Monte Ceneri base tunnels) was costed at 17,76 bn CHF at 1998 prices, so approx 26,6 bn CHF - probably more, because construction costs would have risen quicker than general prices - or a bit more than 21 bn GBP at today’s prices). And that is only a part of what Switzerland has invested in its railways in the last 30 years.
Now Switzerland is obviously a very rich country, but also a small one with about 8 million people.
The IRP has an investment sum of 96 bn GBP, which is about 5 times as much as for NEAT, but for a country with 8 to 9 times more people.
So in effect, at Swiss Levels (and NEAT, as I said, was only a part of total investments - the underground Zurich Long-distance station alone came to about 2 bn CHF, and the planned Lucerne through station will be more than that - the next infrastructure plan, STEP 2035, has a total investment of more than 12 bn CHF) the UK could invest vastly more than it does with IRP.
The thing is, UK people don’t want to - they would certainly not accept a Swiss level of spending for rail, at least that is what I suspect - and the UK government obviously as well, because otherwise they would have spent so much more and thought it a vote-winner.
So my point is: the amount of money spent on the railways, and therefore the outcome, depends on political preferences (and thus on voter preferences). If a country wont support more than a certain amount of investment, obviously not every wish can be catered for.