WordNet records no synonym and no antonym for downing street. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a street of Westminster in London
“the Prime Minister lives at No. 10 Downing Street”
Synonyms, antonyms and the broader and narrower relations for downing street come from Princeton University’s WordNet. Full sources and licences.