It is often fine to say it. However, if you sat down in a works canteen next to a complete stranger and said it to him/her, they might think it a little strange.When I was in the US, I was told "enjoy your meal" several times, so I thought it's all right to use it.
You wouldn't be ridiculed for saying it in BrE, if the context and the situation fitted it. If you were a waiter and you placed the final dish of a course on the table of a group of diners, you might well say "Enjoy your meal" as you quietly back away from the table.
I wouldn't expect to hear it in any informal setting, or from any of my friends or family.
If a boss took his employees out for a meal as a sign of his gratitude, he might say "Enjoy your meal" to everyone once all the dishes are on the table and they are about to eat. Depending on the formality (or otherwise) of the situation, he might say "Right, guys. I'm stumping up for this lot. Tuck in!"
You are reading too much into "commonly accepted standard expression". There is nothing wrong with the words "Enjoy your meal" themselves, it's just that it isn't a phrase which immediately springs to mind. If you asked the majority of BrE speakers what the standard response to "Thank you" is, they would almost all immediately say that it's "You're welcome" (sorry 5jj!). That, therefore, is a commonly accepted standard expression or response. If you asked BrE speakers what they say when someone sneezes, the vast majority of them would tell you that they say "Bless you". That makes it a commonly accepted standard expression.
If you asked BrE speakers what they expect to say/hear at a dining table once everyone is ready to eat, they would not immediately say "Oh, that's easy! Everyone says "Enjoy your meal"." Some of them might come up with that phrase, some would say "Bon appetit", many would say that they can't think of a standard phrase for such a situation.
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