Dansby Parker is almost there with his answer.
As with many cultural differences, this one comes down to Politeness theory. In many cultures, like India and Greece, good social behaviour involves breaking down the boundaries between people you like, such as friends. Sharing a table involves that, and sharing food at the table makes that even clearer. That is called positive politeness.
The land of the British where you find yourself is a land of negative politeness. That means that good social behaviour involves respecting the boundaries between people. That includes the notions of privacy and space, which you must find so puzzling to hear among your English friends. It also includes keeping your food to yourself.
It’s OK, OP. I feel your pain. We Greeks call them cold-arses behind their back. 🙂