By the early 80s the 81-85s were notionally treated as equal to one another for the purposes of allocation to services. However the practical experiences of the Control office staff tended to keep classes 82, 83 and 84 off passenger work wherever possible, as they had proven themselves to be more fragile than classes 81, 85, 86, and 87. The order of preference on class 1 daytime passenger services was pretty much:
1: 87
2: 86
3: 85
4: 81
5: 82 or 83
6: 84
By the early 80s it was comparatively rare to see any 82s or 83s working daytime passenger services, although they did still rack up considerable overnight passenger mileage. This is partly explained by the need for 87s and 86/4s to be taken off passenger work and put into multiple to work overnight freight services in pairs. So there would be fewer of the "preferred" classes of loco available for passenger work, and so the "non-preferred" locos would get their turn. Conversely you would see many Roarers on daytime freight services, which tended to be lighter loaded.
Once the class 90s had arrived and Roarers were life-expired, their retirement was pretty much in the reverse order of the above (as you might expect).
Another complicating factor was that some class 81s and 85s had 80 mph speed limits imposed on them, which restricted their use on passenger services, in an attempt to prolong their useful lives. Naturally that made those locos even rarer on passenger trains. Likewise the unmodified 86s (86/0s) were similarly restricted, pending modification.
So getting an 84 on a passenger train was pretty much only likely to happen if:
1: The Controller was a crank, or new / incompetent; or
2: The bottom of the barrel had been scraped, and nothing else was available; or
3: On a railtour.
