The UK and Ireland have opted out of the Schengen Agreement. The UK wants to maintain its own borders, and Ireland prefers to preserve its free movement arrangement with the UK, called the Common Travel Area. However, they do participate in some aspects of the Schengen agreement, such as the Schengen Information System (SIS).
European Economic Area Citizens do not need entry clearance to enter the UK. The European Economic Area (EEA) includes all EU countries plus Norway, Liechtenstein and Iceland. Swiss nationals also fall under this category. Non-Visa Nationals and members of the common wealth only require an entry certificate when going to the UK to work or to settle. Nationals of the following countries need a visa when staying in the UK longer than six months:
The UK Border Agency offers a helpful guide: Do I need a visa?
Visas are usually issued for six months. If you require a visa and wish to remain in the UK longer than six months, you must leave the country and apply for a new visa. If you wish to leave and return to the UK within the duration of your visa, it will save you a lot of trouble if you apply for a multiple-entry visa. If you only have a single-entry visa you will have to apply for a new visa each time you leave the country.