UK DATV - Evidence onward journey is booked

The text you quote explicitly states that you must provide evidence that your onward journey is booked or confirmed.

So just evidence that the flight exists (or is currently scheduled, more accurately) is not sufficient. You need to show that you have a booking or a ticket. A booking without actually buying the tickets is uncommon these days unless you go through a travel agent (and often, only applicable to specific fares), though some airlines allow it online (sometimes for a fee). In both cases, you should have a PNR (booking reference). If you buy the tickets you will have a ticket number in addition.

As there is no guarantee that you will get your visa, if you actually buy a ticket, you should of course get one that is refundable.

You best bet if you don't want to buy the ticket at this stage is to go through a traditional travel agent who should be able to create a booking for you before buying the ticket. Note however that such bookings may expire quickly, and if the UK authorities check the booking reference and don't find the booking that could play against you. Also, as usual, an actual ticket is a stronger indication that you actually do not intend to enter the UK.

Make sure you double-check the criteria for a DATV. Many types of transits (if you are self-connecting instead of having a single booking for both flights or if you need to change airports, for instance) will require a Visitor in Transit visa instead.