How to find the best-priced UK train ticket combos for not-quite-round trip journeys?

Ultimately there is no system to make this easy, so the way I do it is to learn how to use the Routeing Guide [0] and look manually. It also helps to use a database to let you look at the fares directly rather than trying to find them through journey planners — I find BRFares to be great for this (though it is an unofficial source); there's a similar site LTFares for TfL Oyster/Contactless fares.

The basic process for using the Routeing Guide is as follows:

• First, if the route is the shortest route by rail, or you intend to catch a direct train from the origin on the ticket to the destination on the ticket, and there are no route or TOC restrictions printed on the ticket itself that would prevent these routes, then that route is always valid.
• Second, use the Routeing Point Calculator to decide (via its implementation of the fares check rule) which routeing points are appropriate for the route in question. There's also a way to do this manually but using the calculator is easier.
• Third, look at the Yellow Pages to determine permitted maps to use from any appropriate origin routeing point to any appropriate destination routeing point. If "LONDON" is in the list, you may use maps from the origin to the London Group, then from the London Group to the destination.
• Then, using a chosen set of permitted maps, trace a route using each map exactly once and in the order they are listed, without doubling back (which is defined as passing through a station more than once — there is an exception to this called the Group Stations Rule, which allows you to double back for interchange purposes only through stations part of a single Routeing Group — Routeing Groups can be distinguished by their routeing point name ending in the word "Group"). You may then take the shortest route by rail from the origin to the origin routeing point, and from the destination routeing point to the destination.
• Finally, look through the Eas*m*nts document to see if there are any positive eas*m*nts which permit additional routes, or negative eas*m*nts which restrict your chosen routes.

...But please do read the manuals!

Ultimately with this process you start to get a good feel for what will be allowed and can start to take shortcuts (for instance if you've learnt roughly which routes are valid from a certain routeing point, you can skip a potential destination station entirely if it fails the fares check for the routeing point that you're after).

But more generally, sometimes the following tricks can be helpful:

• Especially with long-distance journeys and with certain TOCs (Southern springs to mind), fares can be surprisingly static even over longer distances, so you can look for locations that let you travel via either route due to the routes having joined back up by that point.

• A lot of fares have quite reasonable flexibility. For instance, while your Vauxhall to Chilworth fare isn't very helpful here, Vauxhall to Sunningdale is valid via both the main line and the Windsor line. You can see this quite plainly if you use the Routeing Point Calculator; it returns all of Woking, Surbiton, and Staines, and because I know the route well I don't even need to bother checking the maps to know that this means you can get as far as Woking on the main line from Vauxhall. So for this I would use Vauxhall to Sunningdale return (£13.60 Weekend Super Off Peak/£15.60 Off Peak) with a Woking to Chilworth single (£5.60 Weekend Super Off Peak/£5.70 Anytime) in this case. I suspect that's probably the best you can do in this case but I could have missed something! Note as well that this would force your route to Guildford to be via Woking; if you need more flexibility there are probably other options (Surbiton to Chilworth single for £8?)

[0]: The railway industry consistently spells the word with an "e", even though this is not the conventional spelling even in British English, so this is the spelling I will use in this post.