It should be spend level per passenger/driver to be correct. That spend level is directly affected by the type of work needed to expand capacity/improve resilience etc., which owing to the current usage of the infrastructure is wildly different between the south-east and much of the rest of the country. As B&I says in his post #512, "This is one of the most irritating aspects of this whole debate. Huge improvements could be made to the most popular lines in the north by spending comparatively small amounts on adequate rolling stock." I am saying that such investment should be made as in national terms, that is the 'low hanging fruit' referred to above. The capacity of some routes could be doubled simply by extending the shorter platforms to allow longer trains. Every time that I have travelled in the north (west) lines I am shocked by the continuing reliance on 3 or even two-car trains calling at platforms that can just about accommodate them. The fix seems to be to run more trains where there aren't adequate paths thereby creating ludicrous delays with overcrowded trains having tedious dwells. That is the sort of problem that needs to be fixed first, relatively easy to do at most locations, then make trains suitable for the expected loads by extending them.
The situation down south is totally different. As mainline commuter route traffic has reached maximum capacity, more has been added by increasing platform lengths to 160m (8-cars), or even 240m (10/12 cars), and adjusting signalling and track formations to permit longer trains to run efficiently on them. Those lines are at the maximum practical capacity* so the only way of addressing increasing demand is to widen formations with more tracks, (rarely viable with land pricies even if any is available), or provide new routes. Doing nothing is not an option as a proportion of passengers will take to the already overloaded roads. Since the car boom started, the south-east has had far less investment in roads (per user) than particularly the north west, West Yorkshire and the north-east, and many of the capital's roads are creating pollution levels well over WHO maxima, so the government cannot just ignore that situation.
*For high density railways, 240m is about the maximum length train that can effectively operate. Even if the platforms allowed, longer trains would produce longer dwells, and longer transit times through junctions. Then there is the problem of locating stabling areas for them. So trains are designed to maximise capacity within existing lengths. Just look at some of the posts complaining about class700s on here.