Kinder Scout in particular has a few Postboxes - these are pre-Geocache and have been going on for quite a while. They norally are either plastic boxes or ammo boxes with a notepad for jottings and often a few related bits of non-perishable stuff. I left a pink elephant in one of them a few weeks ago - if anyone finds said Elephant his name is Tusky.
Karas Box and the box at Golden Clough have become increasingly well-known and unfortunately have suffered through people pinching the box or not looking after it to the point that they have to be removed and dried out. The Kinder River box (which is tricky to find) has a nice rubber stamp in it, but on the past few times I've been there the ink box has been missing / removed. This ones been around since my dad was a regular up there nigh-on 30 years ago.
Karas Box seems perpetually to be removed and vandalised - while the Golden Cl box last time I looked had a mouldy mashed up piece of (yuk) Kendal Mint Cake mashed between the pages of the notebook. The most tricky one to find is the one in the hidden bothy shelter near Sandy Heys.