Of course it's English! Here's the definition from the Cambridge Dictionary. Note... Cambridge. In England. Take a rain check (on something) › used to ​tell someone that you cannot ​accept an ​invitation now, but would like to do so at a ​later ​time: "Mind if I take a ​rain ​check on that ​drink? I have to ​work late ​tonight." The origin is from American sports fixtures. In the past, if spectators paid to attend a sports game of some kind and it was cancelled because of rain, they would sometimes receive a slip of paper ( 'a check') to allow them entry to another game at a later date. It's a nice expression, isn't it? I'm English, but I use it. That is the beauty of having a global language as your mother tongue - the more cultures that contribute to your language, the richer it becomes.

Hi It's an American expression and English people hardly use it. I honestly wouldn't worry about it; if it isn't recognised by the British, it isn't English :D

This expression is probably related to how events get cancelled due to rain. A rain check simply means that you can try again some other time. Expressions like this are frequently odd and can be confusing when taken literally.