I agree, student accommodation is a good place to look-even abroad (i have a friend going to a place near Barcelona who's staying in empty student accommodation), i'm not sure if it's as cheap as you might want though.
You could try a few days in Cornwall/Lake District where you could rent a cottage (again, not the cheapest...but then again you don't want to stay in tents) or you could try a chalet...or is that too much like a tent?
If you really want to go away, try planning and booking in advance-i'm going to Europe for 33days with 3 friends and we started thinking about it in November-January, everything's almost booked and paid for, i leave after my exams and go on the 3rd July-which will be a nice 18th birthday present for myself!