The problem with comparing the 'newness' of rolling stock is that no comparison is made of the differing traffic volumes, the availability of suitable trains from existing stock and the effect that has on procurement, (irresepective of where the requirement is).
The fleets of trains in the south east are huge compared with those in the north. There are nearly 1000 377 coaches in sets of 3,4 & 5, in use on TSGN routes. That is as many as all class 156, 158, and 170's anywhere outside the south east.
So when there is a need to equip a line into London the typical requirement is 300+ coaches of ac EMUs. Where would they come from? There are 96 'not new' ac EMU coaches in ther whole of the Northern franchise, so you could create a micro fleet of 323 and 333 stock in the south-east. Then buy shine new trains to keep you happy. Mmmm, the railway is expensive enough to fund without creating additional inefficiencies just to assuage the jealousy of a few passengers, and opportunism of journalists and politicians.
Just recognise that rail transport in London is all about volume. Over 4 million people travel to London everfy day and rail is the dominant mode as the investment in roads was (thankfully) not extended to the south-east as it was elsewhere in the UK.
The south-east has many examples of electric equivalents to pacers, e.g. tube trains. Most of them are feom the '90s, some are from the early '70s. They have hard seats, they are very cramped and they carry numbers of passengers that sleepy northern commuters just couldn't cope with.