I started to think about this case as if the circumstances didn't involve him moving cities.
If Andrew lived in London instead of Doncaster and was allowed to walk in public alone, if he happened to vanish while walking two blocks between his home and the bus stop after leaving to go to school as usual, you might entertain many scenarios as an investigator, but the number 1 would be the work of serial killer – a crime of opportunity by a stranger or someone he knew and offered him a ride.
But if Andrew lived in London and was allowed to walk in public alone, yet said to his parents he was going to school but instead waited for them to leave the house, went back there, changed clothes and took a train to a different city [let's say Whitby, where his parents originally thought he had gone to], any subsequent foul play would not point as much to a stranger, but to someone he knew and was in cahoots with – or, at the very least, to a private life that the adults close to him were unaware of. The deviation from the routine was too major; it would add a whole new factor in the probabilities for a random crime of opportunity.
I think it's obvious Andrew was killed by someone when he didn’t come home that night or the following days. But a crime of opportunity by someone who randomly approached him that afternoon, relying on no previous relationship with this boy, doesn't sound as likely to me. It could be someone based in London, or in Doncaster or somewhere else who made plans to meet him there. Unfortunately, the trace of online history, potential texts etc are long gone, but I believe the police might be sitting on some stuff that might lead up to something.