Interesting question - I don't think that there's a current British accent that would rhyme those two words, and 1834 seems too late for the Great Vowel Shift t

Interesting question - I don't think that there's a current British accent that would rhyme those two words, and 1834 seems too late for the Great Vowel Shift to have made any difference between then and now. Not every rhyme in the poem is a full (aka "perfect") rhyme; there are a few half rhymes (cold/emerald, thus/albatross). So there's at least an argument that Coleridge didn't intend a full rhyme here.