In British English, adjectives have a comparative and superlative form. Prefixing "more" is 100% incorrect.And “rarer”, I thought that wasn’t really correct and you were to say “more rare”, instead. Perhaps if everyone says it, it’s automatically ok.
Adjective: Rare
Comparative: Rarer
Superlative: Rarest
Not me being "elitist" (a common slur used to excuse bad education or learning) but it always has been here and teaching these basics used to be part of standard, early high school (11+) English Language curriculum. If I hear "more rare" it's jarring and reveals a bit about the speaker.