Manager refused to serve us

Iv been disappointing by this Spur on numerous occasions before but this one definitely takes the cake. We had been looking forward to their delicious double thick malt milkshake which we buy every month end without fail, to have with our lunch which we bring ourselves as we work in the opposite office park. We got there and the waiter knew what we wanted but he returned to tell us that his manager says he cannot serve us as we have our lunch boxes on the table. We then said please can we take it to go of which he went back into the restaurant as we were waiting outside. He returned again to tell us that no unfortunately the manager says he cannot serve us the take away because we have our own food clearly and drinks are only to be served with food. Please can anyone tell me where the hell this rule comes from I have never in my life heard of such. I am very disappointed and hurt to say that we walked away without the milkshake and the manager refused to come and tell us this himself. Spur I need answers!!