Hi Matt,
me and 5 of my mates are going down to cornwall, from London, after our GCSE's too, in mid-July. We booked about 2 weeks ago, and literally went through about 10 different surf lodges, so we got to compare them. But we chose Cribber Green Rooms, it looks really fun, and it's practically on Fistral Beach the web address is :
www.cribbergreenrooms.co.uk
It's between £140 and £150 depending on whether you want sea-view or en-suite etc.
We asked about parental consent, but they said you don't really need it, I suppose everyone going down to Newquay around that time is going to be under 18.
Hope that helps!
Jules.