There were actually two Comets used for sundry training purposes at Gatwick over time, both inevitably ex Dan-Air machines. One was Comet 4 6414, G-APDM, retired in 1973, and the other was Comet 4B 6422, G-APMB, retired the previous year. I can remember seeing the one still there in the 1990s as well.
Using an old aircraft for training the tug drivers seems to have been a BAA thing - there was a Trident at Stansted in BA colours and quite well presented also until the early 1990s, long after they were finally retired from service, which was sometimes up against a gate at the terminal, and must have foxed visiting spotters who saw it from a distance !