Castlerigg Hall has been mentioned for camping. It is a lovely site however you can't book in advance and there are no groups accepted. If you all turn up and start setting up close together it may set a few tongues wagging. It is first come, first served so if you get there late on the Saturday there may be a problem with getting in.
Sorry not sure of other sites in the area, just I have stayed here with the family.
I haven't done an ironman or big (Outlaw) event like that. The swim will be no where near crowded like I've seen on the telly. I think there were about 50 swimming when I did the single which included deca,quin, single and half enduroman enteries. Don't think it will be like that next year as the double starts on the Friday.
If you are pedalling 300 miles at around 18mph you shouldn't have any problems with the biking side.
The run is just keep going and take support of everyone every lap to keep you going.