The article investigates the social relations between Indonesian immigrants and the multicultural Muslim community in London by examining the applicability of the Ummah concept, in the context of the diaspora. The Muslim diaspora, though their similarity of faith, has always contained internal diversity and fragmentation. Likewise, different religious trajectories of Muslim immigrants as illustrated by Indonesians in London have been identified to shape different understandings of unity and diversity of Muslims, which forge different forms of social relation with fellow Muslim immigrants in the city. The traditionalist London Indonesians have trivialized the unity of Muslim in diaspora through daily encounters yet maintained inevitable different ethnic affinities and religious-sectarian affiliations as a wall dividing them altogether. The revivalist Indonesians have construed the diasporic unity of Muslims as an idealized-normative concept that should be realized socially, culturally and politically by suppressing internal ethnic, national and religious-sectarian fragmentations. While the secularist Indonesians have shown an apathetic position to the implausibility of the diasporic unity of Muslims due to its irreconcilable perceived internal diversities and divisions.