and on a trip to Jersey you couldn't have extra diet coke (yuck) because BA return cater the trip and there may not be enough for the return.
Plain illogical.
If someone asks for something, and you have it, then give it to them. If you hold onto stuff in case someone else wants it, you lose automatically on the return leg if someone does not ask for it. You have unnecessarily upset a client just because the airline couldn't get its planning right. And you don't get a consequential delighted customer on the return leg to offset it.
If OTOH, someone does want one on the return leg, and you're out it will be something for the statisticians to learn from overall when stocking up for future flights, and they will be grumpy but understand more than being told you are saving it for someone else who may want it more than them.
To hold back stuff from sale in case someone else wants to buy it later is just plain old silly. Imagine if they did that to you at a checkout at a supermarket !