1261
When you were at the College, there was no question of "duff instructors" or of you not being "allowed" to use such callsigns. That type of callsign was - and is - included in the relevant classroom work and one or two examples were included in the practical exercises. However, most trainees (evidently including you), at that stage, are not astute enough to latch on to that type of callsign that the pilot has initiated and "revert to type" and just use the registration. You would also have been referred to the relevant reference in MATS Part 1.