Here is the Law Society template policy and web page blurb for anyone interested: www.lawsociety.org.uk/en/topics/lgbt-lawyers/transition-and-change-to-gender-expression-template
As pointed out below, one use of "stereotypical" is within the definition offered of "biological sex": "Biological sex
Refers to biological differences that have been stereotypically grouped into male and female categories.
Some of these differences can be found within a person’s gonadal, morphologic (internal and external), chromosomal, and hormonal characteristics.
There’s no one indicator of ‘biological sex’, which is determined using a combination of these factors." (Emphasis added).
Another is within the definition of "Assigned gender/assigned sex":
"The sex assigned to a child at birth and recorded on a child’s birth certificate. Usually based uniquely on a stereotypical analysis of male or female genital appearance.
An assumption is made that the gender will match the sex assigned at birth, so much so that the assigned sex becomes the legal gender of the individual." (Emphasis added)
Given the (dictionary) definition of "stereotype" is "a widely held but fixed and oversimplified image or idea of a particular type of person or thing," and given the dictionary, and widely understood, definitions of "sex", "male", and, "female", I personally think the Law Society guidance is stretching meaning and comprehension to breaking point in the above.