Thanks for your suggestion!
The issue was taken to Ops and Services Advisory Forum on 25th April, and I have a response. This is an abridged version - if you want more detail please check out the minutes!
The main issue with build-a-burger was that it resulted in a high level of wastage of food, as it was difficult to predict what students might go for on any given day. Unfortunately we won't be bringing it back in the near future, but I'd definitely recommend checking out the burgers in Bar 3, or heading to Piazza, who I'm told do a similar offer!
Flora, Vice-President Student Communities and Deputy President