Spithead is an area in the region of Hampton in southern England. This would be a place where naval ships would dock, such as the HMS Thrush, as mentioned in Mansfield Park. Furthermore, this location is likely in Austen's repertoire because of her affiliation with the navy due to her brothers' naval services.

Austen, Jane. Mansfield Park, edited by June Sturrock, Broadview Press, 2003.

The following image is an image of a map showing Spithead. It seems to be more of a region of geographic significance rather than an actual city or established government.