So how come I can't find anywhere to pitch my tent when I'm walking the footways of southern England?
I think you just answered your own question there. Trying heading further north, preferably far north of the border. We a drove through a small chunk of the Highlands last year, the amount of vast, open and unspoilt space up there is quite overwhelming, we are so used to enclosed fields and villages every couple of miles. Anyone who says there are no wild areas left in the UK have clearly never travelled very much around ths country.
Ill give credit to the UK, despite our very high population, we have actually done a pretty good job of retaining many of our green spaces and natural areas. We are decidedly better than a lot of other countries in the world.