raining cats and dogs - heard very rarely, usually if someone wants to make a joke about the weather. It's not the normal everyday way to say it is raining heavily.
I'd normally say 'it's pouring down' or 'it's pouring out there' or 'it's pouring with rain here' or something similar. It's not that important which variation you use.
'Chucking it down' is another UK saying.
If it is really really heavy rain we tend to add 'absolutely' in front of the sayings to add emphasis - it's absolutely pouring today! or it's absolutely chucking it down.