When I was at school in the mid-1970s my trunk was usually sent back home by rail. All the trunks would be packed and piled up near the porter’s lodge, and on the last day of term or the one before a BR or NCL lorry (or two) would pick them up. A day or two later what I think was a BR C&D van would deliver it to the house. I was usually taken back to school at the beginning of term (my parents didn’t want me to do a runner) so my trunk was only sent once or twice. I think you could either take it into a larger BR station, Woking in my case, and hand it over a number of days in advance or you could have it collected from the house, which required more notice and probably cost more.
I suspect that the service probably disappeared in the hurried slaughter of C&D services, which was, if I remember correctly, in mid-1981. It can hardly have been a useful contribution to the balance sheet.
N.b. the above only applies to school trunks, which were, I suspect, not part of the main PLA offer.