Actions by the Executive (Ministers) derive their authority from either Parliament - [Statute or delegated legislation] or from the Royal Prerogative. Dicey once described the latter as "the remaining portion of the Crown's original authority, and is therefore...the name for the residue of discretionary power left at any moment in the hands of the Crown, whether such power be in fact execised by the Queen herself or by her Ministers"In practice most powers are exercised by Ministers - even those personally exercised by the Monarch are done on advice of Ministers, or according to established conventions. The Public Administration Committee in the House of Commons produced a report on the Royal Prerogative http://www.publications.parliament.uk/pa/cm200304/cmselect/cmpubadm/422/422.pdf.
Reform of the Royal Prerogative forms a major plank of the Government's proposals for constitutional renewal. Relevant documents can be accessed at http://governance.justice.gov.uk/
In the GCHQ case (Council of Civil Service Unions v. Minister for the Civil Service [1985] AC 374) the House of Lords held that the Courts could review the use of the Royal Prerogative.
