Celyn is correct to say that the usual answer to the question asked in the OP is that she depicts Frances Stuart, Duchess of Richmond and Lennox. There is no doubt that the image was popularised and fixed by John Roettiers’ design for the 1672 halfpenny which featured a depiction of Britannia loosely based on the Duchess. Roettiers had first depicted her as Britannia five year earlier on the ‘Peace of Breda’ medal and there are two other medals, also dated 1667, which also show her in this way.
However, Britannia as the personification of Britain, whether the country or the island, was older and there are a number of pre-1667 depictions in which most of the elements of the later iconography are already present. This is a print from 1652.
As this print illustrates very well (it’s the frontispiece to an edition of Selden’s Mare Clausum), she has always been particularly associated with the idea of British naval power.