Four rare K8 phone boxes on London Underground stations including at Chorleywood have been given Grade II-listed status by the government for their architectural and historic interest.

The K8s on tube stations were owned by London Underground and housed an internal phone system for station staff. As such, they were painted different colours to the traditional red phone boxes.

Read more in this article in the Guardian. They are also featured in this YouTube video by Henrys Adventures (at 7:20mins).