I had this problem, but thankfully my school was still open. The exams office was doubtful they'd have any of my details still, but lo and behold they had a statement of results stored, which is what I've been using for my proof to college/university.
I would suggest racking your brains and try and remember the exam boards you sat with and giving them a call, they'll know your grades but you'll have to pay for new certificates. If you cannot remember the exam boards, I'd just ring them all :P