The police didn’t say using autopilot was against the law, they said not being in proper control of the car was.
First paragraph of the article.
"A Tesla Model 3 owner in the United Kingdom has been warned by police to not test the Autopilot capability of his electric car again because the feature is still illegal in the region."
"Wiltshire Police are now indicating that Tesla Driver was breaking the law and have formally warned him to not attempt to display the feature’s capabilities again.
“The UK’s current legislation does not allow for their [self-driving cars] use on public roads,” a spokesperson for the Wiltshire Police Department told the BBC"
"The issue with Tesla Driver’s video is that the car was ultimately controlling the movements, and not the operator himself. This is a breach of laws, which require the person behind the wheel to be dictating the vehicle’s moves while it is in operation."
The police are 100% wrong, but they definitely said AP was illegal.