1. Try to find out if the school has got any long-serving staff who were there when you were. The school office might have destroyed all of their old records but individual teachers will almost certainly remember which examination boards were used.
2. If that doesn't work, try contacting old school friends to see if they remember which exam board(s) the school used. If you've lost touch with everyone, consider joining Friends Reunited or simply placing ads on the customer noticeboards of the superstores (Asda, Tesco, etc) in the area around your old school.
3. Look through a list of the examining boards to see if anything 'rings a bell'.
4. Lastly, if all else fails, try contacting each board in turn, until you find the right one. For a school based in London, your best bet might be Edexcel (because they replaced the old University of London Examining Board). Otherwise try AQA (which replaced AEB) and/or OCR (the successor to Oxford & Cambridge)
With respect to points 3 & 4, this should help:
http://norfolkesinet.org.uk/pages/viewpage.asp ?uniqid=3420
Chris