Barrry Doe's Rail Operators Maps would help in this regard and are always worth a look as he updates them when things change and back copies are available on his web site - but he only starts in 2005. From that point on the series allows you to see the changes I think you are looking forI'm looking for TOC maps from the late 90s. I'm primarily interested in TOCs that were later merged or split so I can get a better idea of what services they operated.
It doesn't have anything for some of the TOCs I'm interested in, and I also find it poorly laid out and confusing to navigate.Andrew Smithers' Project Mapping site has plenty of TOC-related resources, alongside much else. https://projectmapping.co.uk/