On its side is fine providing the main switch is at the top and any lighting circuit breakers are mounted above others. Otherwise volt drop will become an issue as it will be highest at the bottom (due to gravity). So to ensure a healthy max. 3% on the lighting circuits, let's see those 6A MCBs next to the main switch chaps. And if that 40A shower circuit at the bottom of the pile is exceeding 5%, pinch the bus bar just above it with your pliers. A small constriction can be enough to reduce the volt drop (a bit like water being restricted down a narrow pipe). IMPORTANT: always knock the power off and wait 5 minutes for it to drain away completely before pinching the bus bar with your pliers. Alternatively, paint the handles of your pliers red and yellow as this combination is proven to repel electricity.
I have seen side mounted boards with the main switch at the bottom, but the voltage just pools around the switch as it can't overcome gravity (volt drop again). Important that your board maintains its IP rating otherwise the voltage can leak out and collect on the ground underneath (otherwise known as 'earth leakage' from the days when houses had earth floors).
Anything else you need to know, just ask.
