To be fair, I don't differentiate between North and South with regards to England. In my experience it's just all inferior to my yardstick, which is Scotland.
Reasons? The access laws, the overcrowding, and the smaller area of hilly terrain. This opinion is based on cycling in the North York Moors, Exmoor, Snowdonia National Park (I know it's Wales) and the waste of time that was Epping Forest. Oh, and the Lakes, nice trails ruined by the stupid access laws and thousands of pensioners tottering.
To be clear though, some MTBing in England is great. Just Scotland is greater.
There is only one divide North/South in Englandshire for me, and it is based on visits to London to be honest. That is that folk are decent everywhere in the world but London. Spiteful place.
Maybe I'll get a flaming from Londoners? Hopefully!