One of my pet peeves is that UK infrastructure focus seems to be about getting people to the centre of London faster, and that just makes the pressure on London greater at excessive costs, and then you start the cycle all over.
You see that even with HS1 and HS2, where instead of focusing on lines towards London it'd have been better to strengthen the triangle of Leeds, Sheffield, Manchester, and pour money into effectively creating a bigger "third city". Then maybe link Birmingham, and purposefully delay any links to London.
The problem with linking everything to London is that while it makes it easier to work from further out, it also makes London ever more attractive.
But even on a more local level, I wish instead of spending in the centre of London, focus shifted to bypassing the centre. E.g. I live in Croydon. Croydon has rail towards multiple towns around London, but anything not on the main lines in towards London doesn't have commuter level rail services running on them, or in some cases have no passenger trains at all.
Which means that many towns are easier reached by taking a detour in towards the centre and out again, which again means there's an incentive for businesses etc. to be closer to the centre. Even when in towards the centre and out again would be the shortest route, it might be beneficial to encourage avoiding the busy routes towards the centre.
Which again means that a whole lot of towns where there are town centres that have huge potential for large increases in density, and potential for lower cost upgrades of other infrastructure, without affecting the green belt, but where the demand is not there for that density increase because of the lack of efficient transport.
Addressing the infrastructure problem by upgrading and committing to commuter level services to connect more of these towns together so they form more viable counter-weights to the city centres they are around would make it a lot easier to address the issues you're mentioning at far lower costs than yet another big infra upgrade like HS2 or another line in London.
Successive UK governments seem to have been more focused on retrospectively managing demand surges than planning ahead, and as a result seems to be inducing demand that drives more surges to be managed and ever higher costs.