Is this idiom used in modern BrE as well? (spending lots of money on Sth, and parties in special, I suppose. Though not sure about other cases than parties.)
It is used in BrE. It doesn't have to be a party, but it's normally for some occasion that is regarded as a little out of the ordinary.
Ethel: Shall we go to the chippy after the film?
Ron: No; let's push the boat out - we'll go to the Chinese take-away.
Chippy (colloquial) = fish and chip shop.