From the start of Gangs of London Season 2, Elliot wanted to take control of London from Sean.
Hence he first wanted Koba dead, which Sean did in episode 8; then, in the showdown, we saw Elliot fighting both Sean and Billy.
Although Billy stabs Elliot multiple times with a screwdriver, giving Sean the upper hand to pour oil on Elliot’s eye and damage his vision but as it turns out, Elliot overpowers both of them and then uses his strength to strangle Sean, but when Billy pleads with him to stop he lets Sean go the end and promises he will going to take everything from him and that’s the reason he is letting him go.
So as per the ending of Gangs of London Season 2, Elliot takes charge of the whole of London alongside Ed and Shannon.
On the other hand, Sean lands in jail because of the murder of Koba, so how would Sean get released from prison in Gangs of London Season 3? Well, only the next season can answer that.
Gangs of London Season 3 is evident as season 2 doesn’t conclude the story. The battle between Sean and Elliot to take charge of London will also be in progress in Season 3.