From TfL via email
It remains the case that from 1 January all taxis must be fitted with an approved device in the passenger compartment of the taxi. We understand through discussions with suppliers that some of them aren’t currently able to meet demand but other approved suppliers do have the stock and capacity to do this.
Vehicles being inspected at one of our inspection centres or on street after 1 January without an approved device fitted in the passenger compartment of the taxi will be dealt with on a case by case basis. If the vehicle owner is able to demonstrate they have made significant efforts to be compliant with the Cab Order then this will be taken into consideration. Given the extended notice period vehicle owners have had to prepare for the mandate (since February 2016) we don’t feel it is appropriate to formally extend the deadline further.