OK, so I'm a Canadian lawyer - though obviously that means I'm qualified in Canada (Alberta) and not the UK. At this sort of level Canadian criminal law is very similar to UK criminal law (we inherited the same law).
So the grounds to arrest are that police must have "reasonable and probable grounds" to believe an offence was committed. So if UK police are arresting the person responsible (curious the OP's article doesn't mention the name, although it's even listed on Johnson's Wiki page) they have more than just a suspicion that a crime was committed.
So the allegation is manslaughter. In order for the Crown to prove manslaughter, the Crown has to prove that the Accused person intended to commit the act - that is the Accused intended to kick or strike Johnson with his skate. If it was a pure accident then no charge is warranted.
But what the Crown doesn't need to prove is an intent to kill. If there is an intent to kill that is murder, not manslaughter. It sadly happens way more often than you think - someone punches another person in the face, they fall down, hit their head, get a brain bleed (or whatever) and die. That's manslaughter.
There is definitely no exception or exclusion to things that happen on ice during a hockey game. There are certainly examples of charges being laid in the past for on-ice incidents - the Todd Bertuzzi - Steve Moore example is perhaps the most famous, but certainly not the only one.
That being said you have to remember the burden of proof. That is - the Crown must prove the offence beyond a reasonable doubt. In this case the sequence is captured on video, so the facts themselves are not in question. In law, we call that the actus reus. The question for any judge however is one of mens rea - what was the Accused's intent or mindset? Did he have a "guilty mind"?
I haven't seen the video. Quite frankly, I don't want to see the video. I've seen enough shitty things in my job I don't need to seek out more. But that's going to be the question - did the Accused intend to kick Johnson with his skate? And in particular - can it be proven beyond a reasonable doubt that he intended to kick Johnson with his skate?