Is it normal to hate when people say "bum" to describe a butt crack??

Don't say anything. You'll just be showing that you're not very worldly. Would you expect a Brit to be annoyed when you call that big storage area in the back of your car a "trunk" when they think of it as the "boot"? Or if an Australian criticizes you when you say you're going to the "john" to take a leak, when they would say it's the "dunny"?

"Bum", to me, encompasses the whole entire bottom.

I hate it when I hear English people using Americanese terms like butt crack or ass. No problem with Americans doing so though.

Wait... People use it to describe just the crack? It's the whole thing, right?

Well I'm English, and I don't think it's a very flattering word either.

crack is rude