The rule here applies to things said in the Chamber of the House of Commons. It is "unparliamentary language" to accuse another member of being a liar. The Speaker can and does banish MPs from the chamber for unretracted accusations using the word "liar". The Member is usually given an opportunity to withdraw the remark first.
Florid terms have been used over the years to get round this ruling, famously Winston Churchill's "terminological inexactitude". MPs are not bound by these conventions outside Parliament, so they can have ding-dong slanging matches in public and spit out the dummy as much as they want.