[Image description: The Doctor saying, “Go ahead!” and attempting to sign the same to Cass. She looks nonplussed.]

The text on the gif says that the Doctor is signing ‘You are beautiful’ but he’s kind of signing ‘star’ and then ‘face’, but honestly he’s just waving his hands about. Signing means co-ordination of facial expression, mouth-shape and hand movements, so (based on my ~six years of BSL) he’s not signing ‘you’re beautiful’.