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Great Britain is a land of gentlemen and horse riding.
Literal
Great-Britain [topic-は] gentlemen [and-と] horse-riding [genitive-の] country [copula-です].
A stereotypical characterization of Britain. グレートブリテン is the katakana transliteration of 'Great Britain.' 紳士 (しんし, 'gentleman') and 乗馬 (じょうば, 'horse riding') are the two defining features cited. The structure is simple: AはBの国です ('A is a country of B'). The sentence reflects a somewhat dated, idealized image of Britain — the kind of characterization found in older textbooks or cultural introductions.