Well, it was the fashion to put a list of destinations on the walls of stations - even to build them in. But they are hard to revise when they get dated, so maybe not ideal now.
More seriously, for Tube-style navigation around the system there are "
RDG‡'s London & the South East" and
TfL» 's "London's Rail & Tube"*. Currently TfL have their bit online as "
Tube and Rail", in their maps collection. RDG/National Rail have their bit, under the snappy title of "
London & South East Map and Network Railcard area of validity map", in their collection of regional network maps.
As a two-sided paper object it has no single name; neither RDG nor TfL will cede priority to allow that. With two sides it can do the denser London part and the sprawl outside. If you want the same thing to extend further west you'll need three-sided paper (or more wall for posters). And a new regional map; there isn't one for Wiltshire in the collection - you need a bigger city!
That style does show train "lines" as if on exclusive tracks. Much of the Tube works that way, and can be shown so even where sharing tracks. Even the tube doesn't restrict a line to one end at each end, so it does not quite guarantee where you need to change. Still less does it help with optional changes to cope with the mix of train routeings actually running when you travel.
But that is, I think, your best bet. Mind you, I suspect the official (or is that marketing?) mind isn't thinking along the lines of big flat paper plans these days. Isn't everyone preferring to peer at/poke their screen these days?
The maps the south London
TOCs▸ offer are more sketchy, showing where the track is not the services (all monochrome currently). Various other people offer them with pretty colours online - but obviously you've no idea how accurate they are.
GWR▸ may or may not have a network map - it's hard to say; they don't even have "plan your journey" as a menu item these days.
The only more specific "trains run from here to here along this route" maps I've sen are the service maps in the
NR» Route Study reports. There won't be one of those for Clapham Junction now because (1) there wasn't a single Route Study for the whole area, and part of one half of CLJ was a full page, and (2) they represented future service levels, not current provision. And they were hard to follow, too.
*Names as in 2019. They may have changed on the printed version,though I'm not even sure it's been printed recently.
