Jewish Identity]. My focus (for personal reasons) is on those who made the move before the middle of the 19th century.
One point I want to raise relates to a topic that came up recently in a comment on my blog post 'English Jewish surnames'. A commenter mentioned that his/her gentile great-grandfather had been lodging with a Jewish family in Birmingham and married a daughter of that family. They moved to Hertfordshire and became Christian spiritualists.
My speculation is that assimilating Jews were often drawn to nonconformist or marginal sects rather than mainstream churches. The 19th-century novelist George Eliot had a sympathetic interest in Jews, and explicitly wrote about them in Daniel Deronda. But I am also thinking of an earlier book of hers, Silas Marner. The book doesn't mention Jews or Judaism but the main character is obviously being presented as a Jew. I won't go into detail but he is described as having an alien appearance and as belonging to a slightly weird, quasi-Christian sect and as being unfamiliar with the rituals of the Church of England. He is a weaver by trade. (Jews were often involved in textile-related businesses, including weaving.)
Another point... Russell is a Norman name and one would normally expect families with this name to trace their roots back to the Norman invaders of the 11th century. But, in following up some Russell ancestors of mine, I came across a reference (in a book called The Families of County Dublin) to some Russells who lived in Dublin in the 18th century and who were merchants and weavers. Significantly, they were neither Catholics nor Episcopalians (the majority of Irish people were Catholic and there was a large Episcopalian minority). These Russells were Quakers.
There were also some notable Russells who were very active in Cork, merchants most of them. At least one was a draper. Some had links with Lisbon and one at least had business interests in (and travelled to) Brazil. This suggests to me that they were Sephardic Jews who had Portuguese roots, and that they may have simply adopted the name Russell and may not have had any connection to the Normans at all. (Russell is sometimes said to be a possible Sephardic name, but I don't have reliable information on this.)
Alternatively Portuguese Jews could have married into an established Russell family of that region, but I have found no evidence of this.