OK, that makes more sense, also legally, in light of the reference I found. It is not a matter of being legally defined as, but rather, considered and punished as if.It is classified as incest for the purposes of a criminal act, which depending on circumstances allows for other similar charges to be applied separately.
The sexual offenders registration can come from a big number of things, that are not related to blood relations, that why I was wondering.
Oh, it is not only the UK one. Someone already joked about the "German blood", remember the tragedy for Queen Victoria was that she had tried to place all members in the family in the thrones around Europe, and pretty much succeeded, but then, WWI was basically a war between cousins.Which is incredibly ironic given the genealogical history of the monarchy
The emophilia of the Zarevich Alexander in Russia, was directly linked to too much inbreeding between closely related people, first degree cousins.
Is the reason there is at least one country I know of, where "incest" is stricly only for blood relations (other crimes can apply if there is a legal tutor/adopted father or mother relation), but that included (and AFAIK still includes) up to first degree cousins, which instead the UK law from 1956 I quoted conveniently ;-) excluded.