The 3-digit suffix for students in the FTS system changed with course progress, as HP pointed out. It was simple but very effective as the first number immediately told you what level of student experience, or competence at (eg) IF, you were dealing with as a QFI or ATCO. 'LOP826' would have been a student from Linton who was flying solo but had not yet passed the Spin/Aeros Check, and 'LOP526' would have them (ISTR) through Basic Handling Test. In practical terms, it meant you could make allowances for unpredictable behaviour in the circuit, especially if it was busy, given that one of the newbies would occasionally throw a circuit so wide it qualified as a navex or perhaps decide to call finals when downwind, just to keep people guessing.
The only time I heard a callsign go badly wrong was on the way to Akrotiri, having bounced off the tanker a couple of times and beaten our way through France, and then the initial contact with Rome . Our formation leader called, in his best fake Italian accent, "Eh! Roma Control, Roma Control..." but then unfortunately followed it with "Uniform 3 Charlie Chequers 1, 2 and 3, FL320" in perfect Epping Forest. The silence was deafening. Several other calls were ignored before we were told to 'Standby', and all subsequent requests for level, heading etc were declined until we got handed on to the next sector. I don't think he tried speaking Italian again.