I dont know much about PSPs i must admit. Does PSP allow you to use the internet and chat with online players? If yes this opens a door of possibilities.
There would be a lot of free Wifi in London, perhaps back in Andrew's hometown as well, perhaps he did not use computers for internet but relied on his PSP?
I wonder if he met this person through an online game or through a chat room using the PSP? I dont know.. if so it would certainly keep his online activity a secret.
Regarding the charger cord I'd bet the other person had their own PSP handheld with a charger and this is how they were ultimately connected with each other.
Imo he did not take his charger for one of two reasons. One; he expected to be back home within 24-48 hours or Two; the other person he went to meet had their own charger and he knew this upon leaving for London. People's minds also change, the longer he was away perhaps the more he wanted to stay.
I am leaning toward online gaming friend or a buddy he chatted with online whom he had a fascination with.
The only other thing i could come up with is that he met someone at a previous concert or event and could contact the person through PSP.
I dont think it was a friend they had known for years, probably more of a recent friend, someone they had known online 3-6 months, as a teen i never revealed the names of my online friends, it came naturally. Some people do meet new people at concerts, keeping in contact i.e email address, online username etc.
Just a thought. Jmo.