What is the female equivalent of a marquess in British nobility?

WalksBySelf, These peerages do still exist and during state occasions, the peers do get dressed up in outlandish robes. But, as you say, not like those in the picture.😁

Tremain White, you can be such a know-all sometimes Tremain.

Peerage titles do still exist; and there is even a book of peerage available, within the Queen's realm -- so that one (or, one who ought be 'in the know'...) may check same.

Player #53553731, I'm pretty sure these peerages still exist. But they don't wear the clothes in that picture any more!