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Sounds stupid but I remember years ago (I've never registered before) a friend got a certificate when he registered his domain. Am I remembering incorrectly?!
Did you friend file and prove trademark usage for the same terms? That might explain the paper. (I think...)
And no, filing a trademark doesn't grant absolute exclusive usage for the domain name-sake, either.
You'll find a scratch off panel on it, which provides a password for use in Nominet's administration site and will let you make limited changes to the domain name, as well as print off a paper registration certificate; Nominet don't send out certificates anymore, so that's as good as you'll get!