score:2
When I plan a journey by train in the UK I use either the National Rail Enquiries website or the Traveline website.
With the National rail enquiries you get a list of results for the train tickets. You will see the prices drop when you get to the 'off peak' time.
You can either search for a single/one way ticket or for a return journey as well, do that last.
In the results page for London trains you see a Travel card fares button.
Use that and the results are for a journey to London, return ticket and a travel card for all public transport in London.
Select an outward journey and the return journeys offered will fit that ticket or it will be indicated that you can use a train only if you buy a more expensive ticket.
The Traveline site is less useful for finding the cheapest tickets but might be better to plan for a door to door journey.
I think you can buy these tickets/travelcards as cheap on the day, by walking up to the ticket window. (I hope people with more experience can confirm it.)