It all depends on your terms of reference.
If speaking as if there is only north and south, then the Midlands probably becomes part of the north.
If talking about England only and having some flexibility, I'd think of south up to about Bedford/Milton Keynes and then Midlands up to about Sheffeld and north from then onwards.
The north-south divide can mean different things. It can refer to London and the Home Counties as south and anywhere beyond Hertfordshire and Bucks as north.
When people say they are from the south, I've heard people mention places likes Northampton that I'd think of as more Midlands, but I guess in relation to Sunderland or Durham, it is the south.
There's north/south of the river Thames too and for lots of people going behind Watford seems outside their comfort zone.
My Dad liked to drive up the M1 and when he got to the big chimneys near Nottingham, he'd say 'we are now in the industrial north'.
People do forget the Midlands as a separate place and seem vague about where it is.....and those people often just call it the north.