Has someone notified Missing People? If someone contacts them and asks them to add, if they say they can't it may elicit some more info, as you suggest, but surely his family should be kept informed if that is the case?
True. I guess it would only be in the case that he was located and didn't wish it to be disclosed to his family? I'd have assumed the Police would have after a certain time if there was lack of progress, but I'm guessing that's not standard practise?
If there is no way he would have attempted the journey onwards or back home without at least going down to the platform and letting his flatmate know , then that sort of thinking also rules out the intentional disappearance during the same circumstances (i.e. leaving a friend down the platform with luggage)
There really aren't that many more options left if we rule out severe illness and foul play too. If you google "knife crime broad daylight", you will sadly see plenty of results, so I would not really say it's that unheard of. I do agree that the chances of it happening right on the A11 are slimmer.
But some others have mentioned here earlier that he was already throwing up as he was walking out of the station. That probably means that he may have made a mess of what he was wearing, and/or the entrance/corridor. They did also mention that when he looked back, he looked as though he was embarrassed people had seen him. If he had "more to give" and quite urgently, there is every chance he could have taken the first road on the right again, away from people, which again increases his chances of external involvement. If you look on street view, there isn't any pub or restaurant in direct sight to the right of station (maybe a bit further down).
If it were me in that situation and I had a choice between entering a tube station with clothes that I had puked on (and maybe even more), or 15 minutes walking home via empty side streets, I would have rather done the latter and texted my friend to meet me there.
You make a good point actually, I'd forgot Bow Road is essentially overground, so messaging him would make a lot more sense and would have worked. My thoughts about going to the platform were based on usually at Underground stations you don't get signal down at the platforms. I guess that makes it even more strange, because he wouldn't have to go there and risk being sick inside to let him know. I'd have thought if he decided that was what he was going to do, that he'd send the message fairly early on having decided to walk or whatever. So if something happened I think it must have happened within close proximity to the area. I thought about the possibility of some form of Taxi or Uber but would he want to risk that if unable to control his stomach? The fees for throwing up and putting the vehicle out of action can be quite big.
Your idea of him going down a side street to throw up further in peace I could see having happened. The Magistrates CCTV I'd hope would be able to indicate that. If there was some kind of random attack down a side road, it'd still be unusual for the body to then be taken IMO. But then this does seem quite unusual in general.