Green Fire engines were kept in depots for use in National Emergencies. Last time I remember them being used was in the late 1970s during a Fire Brigade union strike.
At the time I was a member of RNXS, Bristol unit and for the duration of the strike our shore base, HMS Flying Fox was used as a base for them. They were manned by army regulars who were based in our unit for the duration which if my memory is correct went on for a few weeks.
Not seen one for a long time so I do not know if they are still kept for the next emergency situation. They will be a bit long in the tooth by know.