Ha! Good point about Death Note. It all sounds very melodramatic. Too melodramatic. Andrew doesn't seem like the sinister or cruel type. It just sounds like maybe he struggled to connect with people, or didn't even care to (I don't blame him; school can be rough). But it does sound a bit like he was - if not bulled, then - othered.
I had a classmate in high school who killed himself who was a bit like that. So, maybe I'm just projecting with all this suicide talk.
Sometimes, people who commit suicide do seemingly strange things. If he wanted to disappear before doing it, he definitely didn't stay in London. The chances of being seen jumping in the Thames would be too great. Plus, I think that means he would've had to stay around all day and into the night waiting for a chance.
I think Andrew would have been spotted at least once during that time by someone with superlative observational skills. Yes, most of us our rubbish at spotting and remembering people (I know I am), but there's always that one person who seems to remember details keenly. In a city of millions, everyone blends in, but there are also millions of small interactions that occasionally get remembered.
I haven't heard one credible sighting of Andrew in London outside, possibly, the Pizza Hut one. And even that is by no means solid. For all we know, he walked straight from King's Cross to ThamesLink or Victoria Station and headed out of London within the hour. Then, the radius gets very large.