The nearest campsite to Haweswater seems to be at Pooley Bridge. I used to visit this one with my sprogs when they were little and its great for kids and popular with families (and has a bar and stuff) - this, of course, might mean that its a bit noisy and possibly not very cheap.....
I'm a bit surprised that there isnt a campsite marked on my map at Shap. Coast-to-Coasters may be able to shed light on whether or not its possible to camp anywhere at or near Shap - a pub garden or something like that.
Don't forget to bag Branstree's NE top - point673 1km South of Selside and on the way to Branstree(t) (probably has a name, but I dont know what it is)
and if you're going to use the car park at Mardale Head - it fills up really early and quickly - so you have to be sharpish in getting there.....