Early days yet. The police will arrest for the most serious offence they think possible. Later, possessed of the full evidence, the CPS will have a lesser, or even no, charge brought. And s18 is serious: the maximum is life imprisonment, though even serious examples don't result in that.
s18 [of the Offences Against the Person Act, 1861] is all about intent. The Prosecution have to prove that you did "unlawfully and maliciously cause grievous bodily harm to any person with intent to do them grievous bodily harm" [there is a similar provision for wounding with intent, but this need not concern us here]
Grievous means 'really serious" which would be satisfied by fractured vertebrae. "Maliciusly" adds nothing to "unlawfully" and "with intent" but our Victorian ancestors were fond of using it, notwithstanding that. :)
Intent is established, not by consequence alone, but by other evidence. That the man was seriously injured [if he was; medical evidence may not show that] does not show that you intended the injury. Obviously, someone who throws a bomb at someone else does intend that, evidenced by the mere act, but that is not the ordinary type of assault. Recklessness, negligence, unforeseen consequence, anything less than the specific intent to do really serious bodily harm, is not sufficient. "You can only decide what his intention was by considering all the relevant circumstances and in particular what he did and what he said about it": the direction to the jury in R v Purcell, approved in the Court of Appeal [ reported at 53 Cr App. R. at page 45 (1969), if you are interested]
It's that which is the difficulty for the prosecution, even on the version that you grabbed the man by the neck and threw him to the floor. In any case, it is quite likely that self-defence is runnable on these facts, in that, without being able to weigh to a nicety what was necessary, you did what you felt instinctively to be necessary and reasonable when under a perceived threat of violence (or to keep the peace and stop violence ).