Draft Renewal Order. This requirement stems from the Bill of Rights 1689 which states that - "raising or keeping a standing army within the kingdom in time of peace, unless it be with consent of Parliament, is against law".
From October 2009, the Armed Forces Act 2006 provides a uniform code of law for the Armed Forces and has replaced earlier "Service Discipline Acts" - viz. Army Act 1955, Air Force Act 1955 and the Naval Discipline Act 1957. The process used to change from the old discipline acts to the 2006 Act regime was exceptionally convoluted - (see the Explanatory note to the renewal order).
Addendum: Simon Jenkins writes in The Guardian 9th June that the armed forces should be wrapped up saving the country £45bn per year. I would certainly not go as far as Mr Jenkins since there has to be the capability to defend essential British interests including those abroad. Also, Mr Jenkins does not consider the amount of employment which the defence industry, rightly or wrongly depending on your viewpoint, generates. I think that I will leave it there since (a) this is a law blog and (b) I wondered here and here whether British foreign policy would be all that different under the new government.