car boot sale vs cut-rate sale

car boot sale and cut-rate sale both are nouns.

Word	Noun	Adjective	Verb	Adverb
car boot sale	Yes	No	No	No
cut-rate sale	Yes	No	No	No

As nouns, cut-rate sale is a hypernym of car boot sale; that is, cut-rate sale is a word with a broader meaning than car boot sale:

car boot sale: an outdoor sale at which people sell things from the trunk of their car
cut-rate sale: an occasion (usually brief) for buying at specially reduced prices

Other hypernyms of car boot sale include sale, sales event.

car boot sale (noun)	cut-rate sale (noun)
an outdoor sale at which people sell things from the trunk of their car	an occasion (usually brief) for buying at specially reduced prices