I wouldn't be surprised to find that in the UK it's illegal to ask a candidate "How do you add to our diversity?" because it could be seen as a thinly veiled way to, for example, discover a candidate's sexual preference - something you are outright not allowed to ask about, except in very specific circumstances[1].

(Also, it's an insufferably pretentious question, even if not actually illegal.)

[1] E.g., it might be legally acceptable for a role in a religious organisation.