If you were only allowed one sentence to describe it, then "304s north of Birmingham New Street and 310s south of there" would be pretty accurate, reflecting both the original ordering and actual usage. Towards their end the Cross-City line in Birmingham was electrified, and 304s turned up on that at the beginning as well.
There were of course exceptions. The only trip I ever made to Altrincham on the service from Manchester, around 1980, had a 310 turn up - never saw this again. Likewise just one 304 at the buffers at Euston.
There was also a slightly strange later batch from the late 1970s of just four Class 312, supposedly built for Birmingham area services, and in fact used around there fairly consistently; I don't recall seeing them ever in London or Manchester, but travelling through New Street you often saw one. Looked like a 310 but distinguishable by aluminium surrounds to the cab windows.I think the West Midlands PTE may have had some involvement with their purchase, so they were kept there.