I´m not English, I´m Spanish but I can tell you that the expression "It´s raining cats and dogs" is currently used in the U.K.

I agree, that's an outdated expression. We just say "It's pouring" or also "It's really coming down out there"

Yes it is. I honestly cannot ever recall using it.

a little bit. but some of the expressions used that are modern are also rude, so it is a safe choice another good old fashioned one is "it is raining stair rods"

I still hear people say this in the USA. It may be more common among older individuals. I can't recall a recent movie or TV series where I heard it used, so it may be becoming somewhat old-fashioned but everyone still understands what it means.