AITA for joking that I wished British people were real

I seriously can't believe I'm asking this but here we go.

I was watching a comedic British show with my friend "Carly" and we were having fun and making jokes and stuff like that. I said "I love this show. I wish British people were real." As a joke, obviously. It was supposed to be unserious and stupid but Carly gave me a weird look. She said that's a weird thing to say. I was taken aback because she's seen me make much more weirder jokes than that and I had no idea why she was hung up on this one. But I apologized and told her it was a joke and I obviously didn't think british people are not real. I followed up by saying that on a serious note I like British culture, otherwise i wouldn't be watching this show. She asked how I would have felt if she said Korean people weren't real (I'm Korean) I told her I would also take it as a joke as long as youre not offending me or my food or whatever. She scoffed and just kept watching.

I didn't know this joke could be insensitive. I mean, I know it's kind of a stupid joke, but I didn't know it could offend like that. She's not even British! So I'm doubting myself on whether this was offensive or not.

This is really dumb I know, but i'd appreciate some other opinions on this.