Effective street searches are quite difficult, especially at the moment when a lot of officers feel under pressure by crowds, a lot of the tome with the glass demon grasped in their hand and set to record. If the chap is cuffed to the back there's often a bit of "that'll do until we get him into the dock". Theres a lot of places that you can secrete items that you won't necessarily find them on a quick pat down.
Unfortunately a lot of officers younger in service (and who don't do a lot of hours, perhaps the case with these specials) will try to get things done quickly to minimise their time under this pressure.
Covid adds a complication, I've never been to Croydon but they may insist that "customers" hands are thoroughly washed before going in. Its quite difficult to wash someone's hands if theyre wrapped up their back, and again if someone isnt through there a lot, they may not seen the more inventive solutions. Especially if the detainee is being awkward or aggressive or plain doesn't want them washing.
Awful, tragic events. I can all but guarantee that the arresting and escorting officers will be piling a huge amount of guilt and regret onto themselves, and as much as this needs to be a learning point for everyone involved in the system... They are also going to need a lot of support in dealing with it. If the suspect survives hospital I sincerely hope he is punished as thoroughly as the law allows. We don't need to rough people up, as a country we are better than that.
RIP.