12A was introduced at the time of the Tim Burton Batman to placate parents of 10 year olds desperate to watch slightly tough superhero movies - still very relevant.
If I recall correctly, Tim Burton's Batman was the first to be given a 12 certificate at the cinema - at the time, it was a legally enforceable certificate, like a 15 or 18. The BBFC didn't have 12 ratings for videos, so when it came out on VHS, they upgraded it to a 15. They later introduced the 12 certificate for videos.
12a came about a few years later, reportedly after lobbying by Sony because they didn't want under 12s banned from seeing Spider-Man (the Sam Raimi one).
It's worth remembering that a lot of films are American, and they have a completely different approach to ratings. For a start, they're voluntary for filmakers (films can be released unrated), and only NC-17s stop under 17s from seeing the film. Parents can quite legally take their kids to see R rated films, hence lots of horrified parents after taking their five year olds to see Deadpool...
To be honest, I'm not sure why The Force Awakens is so much higher than the originals. They featured a mix of limbs being lopped off (Ponda Baba, the Wampa, Luke, Darth Vader), choking (Jabba, by an enslaved Leia), murder by the good guys (Han shot first!), torture (the Emperor frazzling Luke with Force lightning), the aftermath of murder of the innocents (the Jawas corpse pile, Aunt Beru and Uncle Lars' charred skeletal remains), Tauntaun entrails, and numerous battles both on the ground and in space.