That was similar to a couple of years ago. I was on the end of platform 3 (or 5) at Paddington with LDECRexile as he wanted to see and ride on the class 8xxs. He was similarly gobsmacked as train after train of 8-car 387s left with most seats occupied, and that was around 15:00 hrs. He is no stranger to London rail stations, but was very surprised at how busy Paddington had become with outer-suburban trains departing so frequent and full in what was really still off-peak time.
Peak time in other areas in the South East can be quite an eye opener as well, if you ever get chance to be at Farnborough Main at about 8am and watch a 12 coach train arrive and the number of people which get off it only to be replaced by a similar number of people getting back on who are heading towards London (most of those getting off have come from the previous 4 stations).
Likewise watch any of the trains arriving into Guildford during the morning peak FROM London where there's 4tph with 10-12 coaches and the sea of people getting off.
Even off peak there's a significant flow of passengers to/from these stations.
Neither of those places are major cities (the Borough of Guildford has a population of circa 100,000 whilst Farnborough is about 1/3 of that and has a second station, which is also a sight to behold with regards to the numbers getting off a 3 coach train during term time). Both would likely benefit significantly from investment in "London" in the form of Crossrail 2 and/or Southern Approach to Heathrow, yet would likely see a jump in rail growth to fill the capacity created.
Dues that mean that we shouldn't invest in the North, certainly not. However lengthening platforms and investing in 6-8 coach trains would probably do it for a bit at a cost of £1bn to £2bn due quite a large area; probably much larger than you'd get by building the Southern Approach to Heathrow which also sits in that price range.