I would never use these expressions. They are Americanisms which don't sit right in British English. That is not to say they are never heard, because they sometimes are, just not usually by speakers of good English.

Australians use these phrases as well

In California we have called a group of our friends "you guys" for decades. If we stop by a table where acquaintences are eating, and we stop to chat, we would end our short visit by saying, "enjoy your meal" or "have a nice day". It's just something to say instead of "goodbye".

Not true, we still use these phrases.... Although I do change have a nice day to I hope you have a lovely day when I'm talking to my customers. People always like to have nice things said to them and these phrases are just about being nice to people. Daisy Rain has used the other common phrase.. no problem.... we all say that.

Well, I wouldn't say 'Enjoy your meal' because I'm not a waitress :) But it wouldn't surprise me if a waitress said it to me. I wouldn't notice if she didn't though. 'Have a nice day' used to be considered a bit fake and American but it's quite common now. I would never say 'you guys' but I suppose a lot of British people would.