Championship regulations and eligible car lists used to be available on the old HSCC website. The new site (whilst much improved in just about every other respect) does not currently contain links to championship regulations. I've got to email them about something else on Monday, and I'll tactfully make them aware of their omission.
In the meantime, older versions of their website are available via the Internet Wayback Machine here...
http://web.archive.org/web/*/http://www.hscc.org.uk
You might be lucky.