When searching "Andrew Gosden" on YouTube, I came across an episode of BBC's Missing from 2011. The episode includes a little bit more CCTV footage of Andrew, just after he had exited the train and before he left the station in Kings Cross. It doesn't show anything much but I just found it interesting as the only CCTV footage I've seen is of Andrew leaving the station. If anyone's interested in viewing the CCTV, it starts at 3:48 and is only a few seconds long. Just from these extra bits of footage, Andrew just doesn't look like he's running away. My personal feeling is that he was meeting up with someone he knew or had come into contact with sometime before his disappearance and they had persuaded him to meet them in London and maybe go to museums (as Andrew enjoyed visiting them) or something and had told him that they'd drop him off home before his parents got back. OR maybe he just decided that he didn't want to go to school and had decided to go on some sort of adventure and met with foul play. The first theory just seems to fit more than the other.
Here's a link to the video for anyone that's interested:
https://www.youtube.com/watch?v=OPR6kB3vc0Q (CCTV footage starts at 3:48).