My understanding is that the 400ft AGL ceiling establishes a "separation" layer/zone between 'our' airspace and the 'normal' floor of "manned aircraft airspace".
UK rules differ somewhat from American rules in that our 400ft to the ground is to the closest ground, that ground need not be directly beneath the drone. The same rule might also apply in Europe.
With regards to VLOS the danger lies in the fact that you can not judge accurately where the drone is NOR what is close to it. You can only 'say' that the drone is somewhere in 'that' direction and that is limited to flights where there is no significant motion across your line of sight.
I suppose you could use the map to give you a direction but I wouldn't want to rely on your/my translation of the indicated direction.
The camera's view is all but useless for assessing what is in the air near the drone, its field of view is too limited and even with objects/bird moving straight towards, or away from, the camera, they are visible on the screen for so short a time that sight of them is momentary and, when caught by surprise, I doubt many people could react quickly enough to take avoiding action.
Try an experiment, set a reasonably high RTH height, say 200 to 300ft, fly the drone out to distance, go reasonably low and stop looking at the drone. Trigger an RTH and after it has had time to climb to RTH height +20 or 30 seconds or so try to find the drone.
Do not look at the screen or map, can you find the drone? I am deaf so I have NO direction sense of hearing to aid me but in circumstances where I have lost sight of the drone and RTH'ed it is quite often within 100m or so of the home point before I can spot it.