Are you a member of the Camping and Caravanning Club? They have numerous small sites located all around the country for a few pounds per night. They are basic sites with fresh drinking water and toilet emptying facilities, some have toilets and showers but not many. However you can make your own shower and wash room with a toilet tent. "Brings back happy memories"
I have a motorhome and use these sites most of the time, except when I go on a "Posh" holiday and use proper sites.
me and a few mates fish all over the place, all we do for accomodation is get the £9.50 tokens out of the sun or the mirror, put down our choices of dates and places we'd like to fish,and sit back and wait,when the holiday is confirmed just contact the local tackle shop for advice on baits and tides etc.
I have no connection to this site, this info above comes from the fact that I am sat with their brochure in front of me. It is a very nice site with all the modern facilities. There is certainly camping available but I don't know about costs for pitching a tent.
I'll be there at the end of August come rain or shine. Can we rely on British weather. There are plenty of charter boats out of Looe and around the area there are some nice rough ground, rocky bays. Try your luck I can recommend it.