www.aat.co.uk
AAT are one of the world's leading membership organisations for accounting professionals, with more than 100,000 members.
www.companieshouse.gov.uk
Companies House offer a wide range services for companies of all sizes.
(i.e. Company formation, web filing and online returns.)
www.hmrc.gov.uk
HMRC is responsible for collecting the bulk of tax revenue. The site contains some really useful information on this site.