。
Fog was a familiar sight in London.
Literal
Fog [topic-は] London [at-で] accustomed-to-seeing scene [copula-だった].
見慣れた is the past attributive form of 見慣れる ('become accustomed to seeing'), a compound of 見る + 慣れる. Used adjectivally here to mean 'familiar, oft-seen.' London's fog is of course a well-known cliche.