If you want to have accommodation for a lot of people cheaply then you might want to look at bunk barns/ bunkhouses either YHA or independents. They will charge perskull pernight so might be easier and cheaper than cottages since you don't have everyone wanting whole weeks. Many will have the option of camping onsite. Here is
http://www.peakpub.co.uk/camping/ one attached to a nice looking pub in Buxton maybe not ideal because of the lack of communal area but just the first that popped up.
I don't see many cottage owners being flexible on startdates and occupancy at the absolute top of the season. You are quite likely to forfeit the deposit if you break the occupancy limits.
Anyway up to you.