And obviously London, the Brighton Main Line and South Eastern Main Line constitute the entire South of England. Meanwhile, down in Devon and Cornwall there are the very same rail buses!....
Then lets not forget that Oxfordshire has new units as does the Chiltern and LTS lines, oh and the Great Eastern and South West Mainlines, in fact, all the mainlines out of London have had new stock at some point since privatisation, with some even being replaced since.
....Taking North as being Birmingham area and up, we have 139, 170, 172, 175, 180, 185, 220, 221, 222, 323, 333, 350s. If we include Scotland we also have 334s, 380s. ....
Birmingham? The north starts at Birmingham? Wow you really have to go some to justify the 'the north gets loads of new stock' argument don't you. Birmingham is
The Midlands. Scotland never has been 'the north'.
139s (the only class on the national network to only need 50% fleet availability) are only found in the midlands. 172s barely go north of Birmingham. The 180s may go to destinations in the north, but they are just there for the trips to London (and are basically cascaded stock, originally bought for use in the south).
The TPE 170s were cascaded from the south (and Birmingham in one case). The Liverpool-Norwich service lost it's 170s to 158s so that Crosscountry could have them (and before you say it, Nottingham is hardly the north either). The only places in the north you will see 175s these days is the relatively small triangle of Manchester, Chester and Crewe the rest of the time they are in Wales, granted they were introduced to routes in the north (and Wales), but the DfT made sure that didn't last.
The 220 and 221s are pretty much country wide, whilst the 222s only go as far north as Sheffield (excepting occasional use to Liverpool from Norwich) on their way from London. Unless you include the yet to be delivered 350s there aren't many of those in the north either, just those on the Crewe-Liverpool runs.
I'm surprised you didn't mention the 390s, although they are only 'new to the north' in the same way as the IEP will be, ie, only in so much as they go to and from the north on services from London.
323s? New? If they are new, the 319s are a couple of years old! They haven't been cascaded yet, but then you could say that about the 142s, 144s, 150s, 153s, 155s, 156s, 158s and 321s as well, but I don't think even you would dare class them as new!
You're only really genuine case for new stock in the north is the 185s and the 333s and that's loads compared to the new stock in the south, isn't it?
....Let's also not forget that the north had Sprinters decades before most of the SE had power door stock.
But we can't let reality get in the way

.
Decades after the Sprinters? Slight exaggeration there. Sprinters were 1985? 313s were 1978? 365s and 165/166s were 1996? Unless the south east is now everything south of Preston Park, that's one decade at most and it's hardly a fair comparison anyway.
How old was the stock they replaced again?
I have forgotten!
And what of the additional stock that didn't replace anything?
Average age of stock in the south that has been replaced? 40? 45? Average age of replaced stock in the north? Have a guess.....