The UK government operates the Business Link website to provide information on starting and running a business in the UK. You may find this the quickest way to find answers about the rules and regulations governing businesses in the UK.
Companies House manages the registration of companies in the UK. Since the law was relaxed in October 2009, a limited company can now be incorporated by a single individual (the requirement for a separate company secretary who is not also the sole director has been abolished). It costs as little as £14 to register a new limited company.
HMRC (Her Majesty's Revenue and Customs) is responsible for taxation of UK businesses and individuals. The HMRC wesbite provides considerable detail about tax obligations for companies and for the self-employed.
Companies are responsible for collecting Income Tax and National Insurance from their employees via the PAYE (pay-as-you-earn) scheme, and must also pay employers' National Insurance contributions.