For the purpose of the UK’s race relations law, “Ethnicity” has been defined by the UK Law Lords in the Mandla case as follows:
“Lord Fraser: For a group to constitute an ethnic group in the sense of the 1976 Act, it must, in my opinion, regard itself, and be regarded by others, as a distinct community by virtue of certain characteristics. Some of these characteristics are essential others are not essential but one or more of them will commonly be found and will help to distinguish the group from the surrounding community.
The conditions which appear to me to be essential are these:
(1) a long shared history, of which the group is conscious as distinguishing it from other groups, and the memory of which it keeps alive
(2) a cultural tradition of its own, including family and social customs and manners, often but not necessarily associated with religious observance. In addition to those two essential characteristics the following characteristics are, in my opinion, relevant:
(3) either a common geographical origin, or descent from a small number of common ancestors
(4) a common language, not necessarily peculiar to the group
(5) a common literature peculiar to the group
(6) a common religion different from that of neighbouring groups or from the general community surrounding it
(7) being a minority or being an oppressed or a dominant group within a larger community, for example a conquered people (say, the inhabitants of England shortly after the Norman conquest) and their conquerors might both be ethnic groups.
A group defined by reference to enough of these characteristics would be capable of including converts, for example, persons who marry into the group, and of excluding apostates. Provided a person who joins the group feels himself or herself to be a member of it, and is accepted by other members, then he is, for the purpose of the 1976 Act, a member.”
Mandla case law observations follow link: