I'm a former police officer and have very mixed views on this.
Reon's head injury must have been incredibly distressing for him and his mum and I completely understand their anger. I don't see any evidence that it was individually racist though - I think the same would have happened had he been white.
We can still consider it institutionally racist because black people are 2.4 likely to be arrested than white people and so they are disproportionately affected.
Officers receive maybe 5 minutes training on head injuries. No one mentioned a fall or a fight before Reon was arrested so officers would not have suspected a head injury in the same way they would had that been mentioned. The odds of a spontaneous brain bleed are very small and although officers will have had an input on strokes, Reon's age would have meant that wasn't at the forefront of their minds. 9x out of 10, a person presenting as Repn did would be doing so as a result of substance intoxication and/or poor mental health. I don't think the general public has any idea how many people with both these factors the police deal with every day. Back when I was a PC probably 4 out of 5 people I arrested. My DH who still works as a Custody Sgt says he literally cannot remember the last time he booked someone in who didn't smoke cannabis and that around 80% of detainees are vulnerable in some way. Reon's presentation in that environment would not stand out unless someone was properly monitoring him, which is where the problem lies. When everyone is at risk, who do you prioritise?
Custody units have minimum staffing/detainee ratios but these assume detainees who need additional monitoring are in the minority. This isn't the case so only those considered particularly risky end up getting constant monitoring. Cops can be called off frontline duties to sit with those at risk but this leaves fewer cops to deal with 999 calls from teams which are already understaffed. The whole thing is just chronically understaffed and contributes to a conveyor belt approach to people where cases such as this are a statistical probability not a one off. Unfortunately, because it works most of the time, the powers that be refuse to acknowledge the underlying problem, condemning a certain number of people to become the cases where it doesn't work.
Reon absolutely should have been on constant obs had a head injury been suspected but it wasn't. The question is why not. Having been on the inside I don't think it was because of Reon's race. No one thought "he may have a head injury but I don't care because he's black". It happened because people don't look further than their immediate involvement and responsibility because of this conveyor belt approach I've described. The cops probably didn't notice anything that unusual about Reon's presentation and knew the Custody Sergeant would ask Reon questions about his health and well-being. Job done, on to the next. The Custody Sergeant only knows what they're told or can see. They cannot send everyone who seems confused or who complains of a headache or being unwell to hospital (well over half of their detainees do) so unless they can see clearly that someone needs to go to hospital NOW they will simply request an HCP to check over a detainee they have concerns about. Risk transferred to the HCP, onto the next prisoner. Unfortunately, it's not unusual for the HCP (who is sub-contracted) to not be on site and take a couple of hours to get there. In this case that nearly resulted in someone dying.
None of this is an excuse BTW but appetite for change needs to directed to the way the police is organised NOT at individual officers. One of the reasons I left the police was because if you try to be that officer who takes personal responsibility for each and every person you deal with (as I did) you will eventually burn out. I was regularly doing 10-20 hours of unpaid overtime per set to manage all the stuff I hadn't been able to do during paid time because of dealing with people properly. Yet I knew that if I didn't do this or missed something because I was human and overwhelmed, someone might be seriously hurt or die and I'd probably be found at fault because the organisation would just point to the policies no one has time to follow and refuse to back me. That's simply not sustainable and after too many years of missing out on my own DCs life, I gave it up. Good cops get worn down or leave and it is one of the reasons why the police keep making the same mistakes - because the causes aren't being addressed. Senior leaders and politicians just blame individual officers for organisational factors that actually push them to behave as they do because it suits their agenda to not spend money on the police.