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Still a small political force, the party campaigning to take Britain out of the European Union has an office on the top floor of a grand building in the heart of London’s political district. And it doesn’t pay a penny in rent.

Though the United Kingdom Independence Party has no representatives in the British Parliament, it does have a dozen members in the European Parliament. And that entitles it to a small space at 32 Smith Square, the London base of two institutions of the European Union that U.K.I.P. wants to leave.

The subversive act of installing itself there fits the mood in Britain, where European integration never fired enthusiasm and where the euro crisis has stirred calls to redefine Britain’s relationship within the 27-nation bloc.