What you're proposing isn't legal, even if your boss / management does nothing in response. By doing nothing, they would increase their risk of a successful sexual harassment lawsuit.
The Equality Act of 2010 defines harassment as
"unwanted conduct related to a relevant protected characteristic, which has the purpose or effect of violating an individual's dignity or creating and intimidating, hostile, degrading, humiliating or offensive environment for that individual."
Harassment is illegal when it is connected with
"a protected characteristic of the employee, such as his or her age, race, national origin, sex, religion or disability."
The existence and permission of mugs utilizing sexual words, such as "cunt," along with your gift, can contribute to the definition of a "pervasive" environment that constitutes violation of the law.
The law regards sexual context even when actions do not specifically target a person, and includes when actions pertain to women in general.
Where "cunt" could marginally be argued in the UK to have meanings other than a woman's genitals, adding a boobs-related example would likely create a compounding effect.