TheUnitedKingdomofGreatBritainandNorthernIreland[13](commonlyknownastheUnitedKingdom,theUK,orBritain)[14]isasovereignstatelocatedoffthenorthwesterncoastofcontinentalEurope.Itisanislandcountry,[15][16]spanninganarchipelagoincludingGreatBritain,thenortheasternpartofIreland,andmanysmallislands.NorthernIrelandistheonlypartoftheUKwithalandborder,sharingitwiththeRepublicofIreland.[17][18]Apartfromthislandborder,theUKissurroundedbytheAtlanticOcean,theNorthSea,theEnglishChannelandtheIrishSea.Thelargestisland,GreatBritain,islinkedtoFrancebytheChannelTunnel.

　　TheUnitedKingdomisaconstitutionalmonarchyandunitarystateconsistingoffourcountries:England,NorthernIreland,ScotlandandWales.[19]ItisgovernedbyaparliamentarysystemwithitsseatofgovernmentinLondon,thecapital,butwiththreedevolvednationaladministrationsinBelfast,CardiffandEdinburgh,thecapitalsofNorthernIreland,WalesandScotlandrespectively.TheChannelIslandbailiwicksofJerseyandGuernsey,andtheIsleofManareCrownDependenciesandnotpartoftheUK.[20]TheUKhasfourteenoverseasterritories,[21]allremnantsoftheBritishEmpire,whichatitsheightin1922encompassedalmostaquarteroftheworld'slandsurface,thelargestempireinhistory.Britishinfluencecancontinuetobeobservedinthelanguage,cultureandlegalsystemsofmanyofitsformercolonies.

　　TheUKisadevelopedcountry,withtheworld'ssixthlargesteconomybynominalGDPandtheseventhlargestbypurchasingpowerparity.[8]Itwastheworld'sfirstindustrialisedcountry[22]andtheworld'sforemostpowerduringthe19thandearly20thcenturies,[23]buttheeconomiccostoftwoworldwarsandthedeclineofitsempireinthelatterhalfofthe20thcenturydiminisheditsleadingroleinglobalaffairs.TheUKneverthelessremainsamajorpowerwithstrongeconomic,cultural,militaryandpoliticalinfluence.Itisanuclearpowerandhasthefourthhighestdefencespendingintheworld.ItisaMemberStateoftheEuropeanUnion,holdsapermanentseatontheUnitedNationsSecurityCouncil,andisamemberoftheCommonwealthofNations,G8,OECD,NATO,andtheWorldTradeOrganization

　　TheUnitedKingdomisaconstitutionalmonarchy:QueenElizabethIIisheadofstateoftheUKaswellasoffifteenotherCommonwealthcountries,puttingtheUKinapersonalunionwiththoseotherstates.TheCrownhassovereigntyovertheCrownDependenciesoftheIsleofManandtheBailiwicksofJerseyandGuernsey,whicharenotpartoftheUnitedKingdomthoughtheUKgovernmentmanagestheirforeignaffairsanddefenceandtheUKParliamenthastheauthoritytolegislateontheirbehalf.

　　SincetheUnitedKingdomisoneofthethreecountriesintheworldtodaythatdoesnothaveacodifiedconstitution,[37]theConstitutionoftheUnitedKingdomconsistsmostlyofwrittensources,includingstatutes,judgemadecaselaw,andinternationaltreaties.Asthereisnotechnicaldifferencebetweenordinarystatutesand"constitutionallaw,"theUKParliamentcanperform"constitutionalreform"simplybypassingActsofParliamentandthushasthepowertochangeorabolishalmostanywrittenorunwrittenelementoftheconstitution.However,noParliamentcanpasslawsthatfutureParliamentscannotchange.[38]

　　TheUKhasaparliamentarygovernmentbasedontheWestminstersy