SiteVisibility is an SEO agency from the UK. It has been in my original best SEO website design list from 2007.
I have followed it and its founders footsteps ever since. I've been also following one of its most important employees over the years. It's creative director or "director of strategy" Kelvin Newman.
Last but not least I have been reading their blog and been pleased with the unique point of views expressed there.
SiteVisibility has by now 30 employees.