There are two obvious problems when trying to compile a list of top import titles for the PSP. The first is the fact that there just aren't that many top games not already available here – the PSP really only came into its own in Japan after the release of Monster Hunter Portable 2 and it's taken a while for that belated success to filter back to publishers.
The second problem is that Sony really doesn't like you importing its games. It's unusual to find Nintendo being more tolerant of such things, but you'll be very lucky if you can find anywhere but eBay that's willing to help you import anything published by Sony – even if it's available in the US.
But overcome such obstacles and there's still a host of great games out there that are unlikely ever to see a UK shop shelf (even more if you can read Japanese – but Pocket Gamer has left out all the exclusive role-players with the assumption that most people probably can't).