This could be the reason for British accents:
A co-production between HBO and the BBC, ROME is one of the largest co-production deals ever by the BBC for an American series, and marks the first series co-production of the two networks. HBO and the BBC previously partnered on the 2001 miniseries "Band of Brothers," which won six Emmy® Awards, including Outstanding Miniseries.
http://www.hbo.com/rome/about/