If you're eating in the Queen's presence, you best hope she's not just having a light snack. It is considered impolite to eat in the presence of the Queen (or any Monarch) after they've stopped eating. When she lays down the knife and fork, that signifies mealtime is also over for you.
The current Queen is quite considerate about this, and eats slowly, but Queen Victoria was known to be able to consume seven courses in thirty minutes; in some cases, before the final guests had even been served their own meals. That meant the plates had to be taken away as soon as they were laid down, without being touched. Imagine how frustrating it would be to get an invite to a royal banquet, and then not be able to sample any of the food?