I think you mean to say "Although the rhetorical theories that follow are often labeled "British," most of the thinkers described are Scottish, and one Irish. British should not be taken to mean English when applied to eighteen-century rhetorical theory.".
Is it the geographical/historical issues you don't understand, or the language ones? To address the former, the Kingdom of Great Britain in the eighteenth century included both Scotland and England, as well as Wales. (Great Britain is the island which is shared between those three countries, and "British" refers to that land as a whole.) But Ireland at that time shared the same kings, so it was brought within the same sphere of influence. However, technically, the Scots for most of the 18th century were British, but the Irish were not. (They became so in the 19th century.) The English were at all times considered British, but the reverse is not true. It is still the case today that "English" should not be confused with "British". When the author says "British should not be taken to mean English", that is what they mean.