To Watershed's point: "If so, in principle you can just use a through return ticket for the whole journey, since Manchester-Coventry-London is a valid route from Manchester to London. "Any permitted" walk-up tickets for this journey allow you to break your journey."
You don't appear to have answered the question about where you are spending the night, in Coventry or London? This is critical to what options are available to you.
With something like an off-peak return, you are typically allowed to break journey on both outward and return journeys. However, the outward journey must be completed the same day, so this type of ticket is not much use for an overnight break of journey. The return part of the ticket can be broken overnight (or even longer), as it has a 30 day validity. Note there are exceptions to this, some tickets do not allow break of journey.
With an any time return, the outward portion usually has a three day validity, so can be used to break journey over night. But these types of ticket can be very expensive.
The options I would have considered for this kind of three legged journey are:
1. Separate return tickets Manchester to Coventry, and Coventry to London
2. One return ticket Manchester to London, and a separate single Coventry to London
3. Separate singles Manchester -Coventry, Coventry - London, London - Manchester
Which is cheapest will depend on what times you are travelling, and whether suitable advances are available or not. Note that option 1 would require your return journey to be on a train that calls at Coventry.
If you are making another trip from Manchester to London before this trip, another option might be to buy two return tickets for this first trip (one return Manchester to London, and another London to Manchester), using the outward portions for that trip, and the return portions for the trip where you are breaking journey at Coventry. By using the return portion of a ticket for your Manchester to London trip, this would allow you to break journey at Coventry overnight.
Your journey is further complicated by there being alternative routes from Manchester to London, some via Coventry and some not. So for the outward leg you would need tickets valid via Coventry, while for the return journey you may not want to tie yourself to tickets that require you to take the slow route via Coventry. As previous posters have said, you can use this forum's ticketing site to find tickets that are valid by both routes, as it allows you to specify different via points for the outward and return journeys.
But when breaking journey, be careful of any restrictions on the ticket, such as restrictions on the departure times from Manchester or arrival times into London. If you are intending to travel down to London straight after your meeting in Coventry, you might regret buying a ticket that does not allow arrival into Euston between 16:00 and 19:30, for example.
Also, when breaking journey, especially overnight, I would always recommend buying a traditional card ticket. With e tickets, attempting to use the same ticket again on a different day can be problematic. Unfortunately this forum's ticketing site only appears to issue e tickets if you specify Euston or London Terminals as the destination.
With this kind of journey with alternative routes, looking at tickets to/from a common point (such as Rugby in this case, as suggested previously), can also throw up useful savings.
In summary, while you might think your journey not particularly strange, it is not a typical journey. There are a number of possible options for doing it, which is why journey planners don't give a simple answer. Added to which, as spending the night in Coventry may be significantly cheaper than spending the night in London, it might be worth spending slightly more on tickets if that allows you to spend significantly less on accommodation. No journey planner is going to answer that for you!
