I did use HPIcheck for a while, but the email address bit made me switch to Totalcarcheck instead.

Also, for dutch plates, I've discovered Voertuig.net a few months ago. You can search for plates if you're missing the last digits, which is great. And contrary to the official rdw.nl site, it has data for old plates that aren't in use anymore. Or is it "had data" ? Somehow, I haven't found as much info on older plates as I used to.