Apoligies I've got my companies mixed up, I've done a lot of quotes today. I'll correct the OP.
Its ETA, they confirmed on the phone it's 500m. Their policy documents state it is a mile but both limits they said refer to cycle rescue not theft.
https://www.eta.co.uk/bicycle-insurance/key-facts-documents/
With Yellow Jersey their document specifically lists not covered for theft within walking distance, which is nonsense.
https://www.yellowjersey.co.uk/info-zone/
"Pedal cycles left unattended for more than 1 hour within walking distance of your home are not covered for theft, unless stored in an insured location."