I bet I was there! I can remember more than one large convention at the old Twickenham before it was rebuilt. Yes we had the large food catering with dinners on trays . It was fun ,as I recall.
I thought they had camping some time after that . I definitely slept in a large tent with a lot of others actually in the grounds of the stadium. I heard of off site camping from others later on too.
Happy days , when conventions were fun!