Hi Rick
Headcodes have become much more varied in the past few years, particularly for Engineers traffic. The main problem with using letters years ago was the old telephone dial train describers in the boxes. There was only 0 - 9 numbers so therefore only 10 letters available. From memeory they were A, B, C, E, F, M, O, V, X, Z. Once keyboards came into use, the whole alphabet was freed up.
Tampers now are J's and each machine has its own identity, so 6J45 will be allocated to a certain machine, and stay with it. The exception are non-reliable Tampers that dont activate track-circuits. These are still 6Z09, 7Z09. Signallers have to deal with thses moves differently.
Engineers trips were 6Txx until relatively recently (T for tripper I suppose), But the Western tends to use W nowadays. Ys apply for the high output trains.
I'm still amused that everything down the Lickey bank to poor old Gloucester panel is still a V after all these years, the inter-regional headcoses dont seem to have altered much.