Yeah, the lack of details in what the police have released makes it difficult (not saying we are entitled to know, just observing).
The men were detained on suspicion of kidnapping and human trafficking, and the older one was also detained on suspicion of possession of indecent images of children. So, I have to assume that none of the counts have been confirmed as of yet, that includes the images, otherwise they wouldn't still be out there (one would hope...).
In that case, how would they be able to link it to Andrew? Is it because the tip-off they received specifically mentioned Andrew? That's the only reason I can think of, now that we know it was an anonymous tip-off.
I'd be interested in understanding whether their assumption that examining the devices could take up to at least 12 months meant they actually found encrypted data, or that it was just the time it would take to verify whether there was anything suspicious at all on them. I'm assuming it's the former.
On a personal note, I have to say that the fact that these guys are from London and have been released, meaning they could be sitting next to me on a park bench, or on a bus, or on a train, freaks the hell out of me. It's made me extremely anxious when out and about in London.