The Interrail map is only a summary map, it has nowhere near the detail needed to show all lines that are there.
Being absent from the map does not mean a line is outside the pass validity.
In UK all national network lines are valid. Only Metros and heritage/tourist railways are outside the pass validity.
Here is a map with all routes in Britain, note it also has metros as thinner lines (such as London Underground and Manchester Metrolink) that are not valid with interrail.
https://www.nationalrail.co.uk/Blue%20route%20ASandMetro%20map%20v24.pdf
Lots of other maps of British Railways
https://www.nationalrail.co.uk/stations_destinations/rail-maps.aspx
For that area check out Great Western Railway website.
A handy one is also https://www.openrailwaymap.org/
It doesn’t give you an overview on which trains are running where and how frequent, but at least it gives you a complete overview of all possible options where you can get by train. :)