although you anonymously purchase the oyster card, you can be de-anonymized the moment you scan the card, as face recognition links you with that card.
Sure, but the link to an ephemeral Oyster card doesn't give them anything that the face recognition doesn't give them already. If they can track faces, then they can track journeys even if you're paying in cash directly for every ticket.
What happens if you cover your face while you scan?
you may slip up, or the system can always flag the id card if they're unable to make an automatic link to a card bought with cash, causing a human operator to get involved. If they see that you block your face everytime, they then can activate and access the total surveillance resources and flag the card.