I just read through the thread, and many of the links.
Andrew's dad sounds like a lovely man- I cant begin to imagine what his family has been through over all of these years.
Many of my thoughts echo those already expressed.
1. I don't think he ran away. If for no other reason, he left the 100 pounds in his bedroom. He waited until his parents and sister left, returned home, got changed, grabbed a few things and left. He knew he had the house to himself- so why not use that time to pack a few essentials, like the cash, charger and a few changes of clothes? I feel as though this was a day trip he thought he could get away with. The school's policy of calling parents if their child didn't arrive for class worried me a bit, because if he knew about that he would have known he would be busted, but if he had previously had a 100% attendance rate, he may well have had no idea someone would attempt to call his family. He wasn't hiding- he withdrew money using an atm. He didn't wear a cap to disguise his appearance on cctv. Doesn't sound like someone who was about to deliberately disappear.
2. I don't believe he was going to visit family. Too big a chance of being caught. Also- his parents had encouraged him to travel to London to see family in the very recent holidays, and he declined.
3. Andrew walked home from school a few times in the weeks before he disappeared. School had only been back in for eight days, so it must have been just before he disappeared. To me, this must be related to his disappearance. It sounds as though he was meeting up with someone on the way home. Question: What was Andrew doing during the day, during the holidays? Were his parents both at work? Who was he hanging out with? Andrew loved London, by all accounts, but when his parents suggested he go there during the holidays he declined. Why?? My gut says because he was hanging out with someone locally. Someone he then met up with a few times on his way home from school.
4. My theories: a) Andrews friend, whoever they were, travelled to London before Andrew, possibly the day before. Andrew planned to meet this person in London, for a reason unknown but that required more than 100 pounds but not more than 180 pounds. He did not need a return ticket because this person would be driving him home (making me believe that whoever it was must have been at least a few years older, to have a car and a licence).
b) Andrew travelled to London on behalf of his friend, to meet someone/deliver something to someone.
I believe Andrew had every intention of eating dinner with his family that night.
I've always been convinced the keys to this case lie locally and not London. I'm with you, I think whatever plan was hatched stemmed from someone he met or hung out with over the Summer holidays.
It sounds as though his parents were out at work when he usually got home from school, hence they assumed he was somewhere in the house the day he disappeared. I'm speculating but it's not unreasonable to assume they were out most of the day over the Summer too. Plenty of time for Andrew to hang out with/talk to/plan with his new friend.