Hi there,
As mentioned previously the Quail Track diagrams are good. You can read about them here..
https://www.trackmaps.co.uk/trackmaps/railway-maps/railway-track-diagrams/
They are regional, whereas the Trackatlas covers the whole of the Uk. See Amazon..
.https://www.trackmaps.co.uk/trackmaps/railway-maps/trackatlas/
For technical information, Nitwit Rule publishes the Sectional Appendix online for free. Find these here..
.http://archive.nr.co.uk/browseDirectory.aspx?root=&dir=%5csectional%20appe ndix%5cSectional%20Appendix%20full%20PDF%20copies
They're very detailed though, and are like zooming in with a microscope. If you're not familiar with the terminology you might find them a tad daunting, but they're Freebies so it's worth the risk.
HTH, BW