The BR regions had considerable autonomy in the early days, ....
The class 50s that you think of as "Western" actually began life as an LMR project to accelerate Euston-Glasgow trains on the WCML (they are advanced class 40s underneath).
After Crewe-Glasgow was electrified, they were sent to the WR to replace the diesel-hydraulics that were withdrawn en masse at about the same time.
That episode saw the end of WR autonomy in traction matters.
....
The 50s began to be sent to the WR in about 73 or 74. The WR had already been forced to accept scores of Cl 47s from 1963 (?) and Cl 37s from, er not exactly sure - probably c 1964.
The 'ubiquitous factor' was a historical process. When the 30/31s first came out, they were all on the ER. You never got them on the LMR (except at 'border' stations like Leicester or Northampton) or on the Western.
When BR decided to keep such classes in service, and many of their original turns (or lines) disappeared, so they began their regional travels.
In steam days, thousands of locos rarely got beyond 100 miles from their home depots. On the GN, for example, Gateshead (and, especially, Scottish-based) pacifics were rare at the southern end of the line, because most trains changed locos at Grantham and Newcastle. (The Elizabethan loco being the exception to this rule in the case of Haymarket A4s.)
And in many cases, shunting locos rarely strayed more than 5 miles from their local MPDs, except for overhaul. Same for many specialist classes - eg the S&D 7Fs.
Of course, 'standard' designs, like Black 5s and 8Fs, worked over areas from Scotland to Swansea - just the individual locos did not stray. So, had the OP gone spotting at Bedford in 1960, he would have seen a Jubilee like "Hong Kong" three times a week (it was a Kentish Town engine, I think) - but never caught a sight of Mars (a Bank Hall loco) or the Corkerhill or Polmadie locos (not sure exactly, but many of the 57xx series were Scottish based IIRC) in a month of Sundays.