Those gears look fine to me, the 'dog' teeth (the internal small teeth and those on the sliding selector gear) on mine were more worn than yours and my box stays in gear no problem.
The following may help.
Have a look at this Youtube video I did re the BA. https://www.youtube.com/watch?v=plr2YoD ... dex=5&t=0s
As shown, the main gears themselves do not move only the selectors with the 'dog teeth' move.
At about 11 mins i show the box going into second gear. As you can see the sliding selector gear has to move quite a way to engage the 2nd gear so, for it to 'jump' out of gear the sliding gear must be moving out of mesh.
For each gear position the sliding selector is held in place by the bronze selector fork which itself is held in place by the pin/dowel in the camshaft slot.
The camshaft is held in each gear position by the spring loaded pawl, the pointed 'tooth' of which sits in one of 5 machined 'V' grooves around the camshaft. There is a separate 'V' for each gear position and neutral.
So as long as the pawl is firmly seated in its groove (and the spring is doing its job holding it there) the camshaft cannot rotate any further.
The camshaft is rotated into each gear position by the gear change mechanism in the outer cover. When operated by the gear lever the mechanism needs to rotate the camshaft sufficient for the Pawl to fully engage in the 'V' groove in the camshaft.
So thats how it should work. Common areas where wear can cause jumping out of gear are :
Weak pawl spring or worn pawl
Worn selector fork/camshaft slot/pin
wear in the gear change mechanism resulting in the camshaft not being turned enough for the pawl to fully seat in the 'V' groove (see attached PDF (last page) which explains where this wear can occur)
Hope this is of some help