I don't usually look at the Japanese Playstation Store much but there is a demo I'd like to point users to. If you create a Japanese Playstation Store account and activate your PSP to point to the new store, you can get hold of two interesting demos. One is "Prinny: Is it OK for me to be the Main Character?" while there is also a demo of the newly released (in the US) follow up to Riviera called Yggdra Union. I don't expect much of a language problem on Prinny but Yggdra Union may be a little text heavy.
Oh, and unless Sony haven't told me something, you should be able to switch between your UK and Japanese PSN accounts easily. I would do it like this:
1. Deactivate your PSP from the UK store (You will lose access to your UK downloaded items until you reactivate)
2. Clear your store.playstation.com cookies from your browser:
In Firefox, this can be done by going to Tools > Options and selecting the Privacy icon from the top menu. Click "Show Cookies" and search for "store.playstation.com". You should see 2 cookies. Click on the first one, hold down the Ctrl or shift key on your keyboard and click on the second cookie. Then click on "Remove cookies" and close/OK all the dialog boxes until you're back at the browser. Head to the Playstation Store and it will ask you to select your region. Voila!
For Internet Explorer, click on Tools > Options. On the general tab, under browsing history, click on "Settings", then "View Files". Look for the file "cookie:???@store.playstation.com" and delete it (where ??? is your username I think). Then, just head to the Playstation Store.
3. Go to store.playstation.com. On the country selection, choose Japan.
4. Create a Japanese PSN account (there should be a couple of guides floating round)
5. Log in to your new PSN account.
6. In the Japanese Store, just download one of the demos to activate your PSP to the Japanese account (as long as you're at the latest firmware and version of the Playstation Network Downloader, you'll be fine).
7. To change back to the UK store, re-do step 2 and then select UK. Then just re-activate your PSP.