They changed this some years ago, and the new interface is far less functional. (I used to routinely book in the same way you've described) There is no way to make it show the flights directly that I know of, unlike the old style interface that showed every possible option and made it easy to do what you wanted.

Possible workarounds:

Use Google Flights and see if it will let you click through to purchase on ba.com

Use ITAMatrix along with the relevant PowerTools plugin. I've had success with this in the past.

Book a flight you don't want but it is offering (e.g. the morning one), then phone up within 24 hours to make a free change to the one you do want (i.e. the evening one). This seems like madness to me I admit, but it's been recommended here in the past. You could of course also just book the whole itinerary on the phone, but presumably the idea is that changing just one flight is simpler. If you take this approach, use e.g. Google Flights beforehand to get pricing info.