Class 300 was allocated to a proposed fleet of parcels-carrying Electric multiple units, which were to be converted from former Class 307 units.

In the early 1990s, the Class 307 passenger units were being withdrawn from service. At the same time, the Parcels sector of British Rail were looking for a fleet of parcels multiple units, which would be more cost effective to run than locomotive_hauled stock as previously used. One suggestion was to convert a number of Class 307 to meet the need, as had been done with four of the similar Class 302 units, and several units were set aside and stored with this possibility in mind.

However, the age and deisgn of the units counted against them, and the decision was taken instead to build new units, which became the Class 325. However, the driving cars from the Class 307s were still used by the Parcels sector. They were converted into Propelling Control Vehicles (PCV) and used as a kind of driving trailer car at the end of locomotive-hauled parcels trains.