Unquote #30 answer

The quote

And the London Underground is not a political movement!

The puzzle as it was set

OZS BTY CUZSUZ IZSYKFKUIZS JV ZUB O GUCJBJMOC AUDYAYZB!

The key

Each code letter stood for one real letter (code letter, then the real letter it hid):

O → AM → CS → DY → EF → GT → HJ → IC → LA → MZ → NU → OG → PK → RV → SB → TI → UD → V

The most frequent code letter was Z (7 times), which stood for N. Short words, apostrophes and repeated letters are the usual way in: a one-letter word is almost always A or I, and THE is the commonest three-letter word in English.