"all formal contemporary notions of female fashion"
Example of artistic dress in 'The Blue Silk Dress' by Rossetti
Public DomainExample of artistic dress in 'The Blue Silk Dress' by Rossetti

The clothes Fowles describe almost but not quite fit into the genre of Artistic Dress, the name coined by the Pre-Raphaelite school for the styles worn by their female associates, which were heavily influenced by the clothing depicted in Pre-Raphaelite art.  The rejection of all fashionable conventions is deliberate: these conventions were regarded by this group as either ugly, impractical, uncomfortable or unhealthy.