The RSPH Level 3 Diploma in Healthcare Science (Anatomical Pathology Technology) is the qualification which is studied by Trainee APTs
This course is recommended to be started after around 1 year of beginning a trainee position and takes at least 18 months to complete (it does take longer in a number of cases, especially if during a pandemic!).
I have had quite a few questions and requests to discuss the Diploma in more detail in comments or on Twitter, so I will do my best to answer the questions here.