In reply to tehmarks:
Not the north but large parts of Dartmoor meet your criteria.
Wild camping is allowed and if stray more than a mile from an ice cream van you are likely to have the place to yourself for much of the day.
Developed areas you could look at would be Coombeshead Tor, Sheep’s Tor and Cuckoo Rock on the SW side of the moor.
On the north moor I’d look at Tavy Cleave, Great Links Tor, West Mill Tor and Irishman’s Wall. For real isolation get yourself out to Fur Tor.