Paul Middleton originally emailed a number of companies yesterday who probably registered a strong interest in exhibiting back in October. A source forwarded the email to us and we ran the above article because of the public interest surrounding the roadshow and its news value. Earlier today, Paul sent out a press release, which other outlets have picked up. A company that wants to exhibit should probably book a place asap, from what I can tell.
As for publicity, I write for local newspapers and getting an article in about the risc os roadshow will be difficult unless you can spin it hard on retro gaming or something leftfield like that. I really recommend contacting surrounding user groups and computer clubs and spreading the word to those who are likely to be interested - targeted publicity.