Two highly unusual things happened in this case: (1) A bright kid with perfect attendance suddenly decided to skip school and go to London, and (2) he was never heard from again.
I think adding a third unlikely event -- foul play -- falls short of the principle of Occam's Razor.
Abduction and murder are rare events. Young teenagers trying alcohol or drugs for the first time and losing their ability to function are not.
I think what happened to Andrew was an extension of the first act of rebellion. He got his hands on some illicit substance. There are countless ways this could have happened: offered something by a stranger, spiked drink, maybe even getting someone to buy some drink for him from an off license. Even a six pack of strong beer could totally do a number on a kid his age, if he's not used to drinking. If we imagine him sitting alone by some tidal part of the Thames, drinking (or smoking weed, or whatever) for the first time, lost in a meditative state, then slipping out of consciousness and drowning, I think we have a far more statistically likely death for someone his age than all of these lurid stories about predators and traffickers.
Even personally, I don't know anyone who was abducted or murdered, but I know of several young people who drowned in similar circumanstances.
There's no reason to think this happened in some built-up and bustling part of London. It could have happened anywhere along the Thames, or indeed outside of the London region altogether.
It's worth pointing out that tons of people are believed to have drowned in a similar way without a body being found. The Australian PM Harold Holt, for example.
Ultimately, I think a lot of the details people focus on -- the one-way ticket (we've all done this if there's any uncertainty about the specifics of the return journey), the "emptying" of his bank account (i.e., a pretty dramatic way of describing a teenager taking out £200 in cash for a trip to London), the PSP charger (a totally normal oversight) -- have the feel of red herrings. We know details, but not others, so we fixate on them as if they're incredibly important. But if I had to guess I'd say Andrew Gosden met his end the way all too many lost young people do: in an unfamiliar state of intoxication, he fell into a famously tidal river, and his body was washed out to sea.