There's plenty of political jokes centered on the fun idea of UK offering wayward colonials re-admittance into the Empire in view of them being clearly unable to govern themselves for one hot-topic-of-the-day or another (due to timing, most of the ones I heard were centered around GWBush elections).
Jokes aside, what exactly would be needed for a country which is not currently a member of The Commonwealth, to join Commonwealth of Nations?
The current membership criteria is established in the Kampala Communique, passed at the 2007 Commonwealth Head of Governments Meeting. The current criteria have already dropped the requirement that members must be formerly part of British Empire. In fact, two Commonwealth members have never been a part of the British Empire at any point of history - they are Mozambique (a former Portuguese colony) and Rwanda (formerly part of German East Africa).
Heads of Government reviewed the recommendations of the Committee on Commonwealth Membership and agreed on the following core criteria for Membership:
(a) an applicant country should, as a general rule, have had a historic constitutional association with an existing Commonwealth member, save in exceptional circumstances;
(b) in exceptional circumstances, applications should be considered on a case-bycase basis;
(c) an applicant country should accept and comply with Commonwealth fundamental values, principles, and priorities as set out in the 1971 Declaration of Commonwealth Principles and contained in other subsequent Declarations;
(d) an applicant country must demonstrate commitment to: democracy and democratic processes, including free and fair elections and representative legislatures; the rule of law and independence of the judiciary; good governance, including a well-trained public service and transparent public accounts; and protection of human rights, freedom of expression, and equality of opportunity;
(e) an applicant country should accept Commonwealth norms and conventions, such as the use of the English language as the medium of inter-Commonwealth relations, and acknowledge Queen Elizabeth II as the Head of the Commonwealth; and
(f) new members should be encouraged to join the Commonwealth Foundation, and to promote vigorous civil society and business organisations within their countries, and to foster participatory democracy through regular civil society consultations.
The Requirements for entrance to the Commonwealth of Nations are contained here.
The criteria have been altered by a series of documents issued over the past eighty-two years.
The most important of these documents were the Statute of Westminster (1931), the London Declaration (1949), the Singapore Declaration (1971), the Harare Declaration (1991), the Millbrook Commonwealth Action Programme (1995), the Edinburgh Declaration (1997), and the Kampala Communiqué (2007). New members of the Commonwealth must abide by certain criteria that arose from these documents, the most important of which are the Harare principles and the Edinburgh criteria.
The following states would be eligible under the Edinburgh criteria (but not necessarily under the Harare criteria). The first list is former colonies, protectorates, mandates and dominions of the British Empire and do have some constitutional link where some or all of this nations territory at one point was under British control.
Bahrain: British protectorate until 1971.
Egypt:British protectorate from 1882 until 1922 but British troops still defended Egypt in the Second World War and helped train and controlled the Egyptian Army. British troops controlled the Suez Canal until 1956; English commonly used as a language of instruction and administration.
Eritrea:administered by Britain under a UN Mandate until 1951.
The Gambia: member of the Commonwealth until its withdrawal in 2013. However, newly-elected President Adama Barrow has pledged to return The Gambia to the Commonwealth.
Iraq:British Mandate of Mesopotamia until 1932.
Ireland: the only country ever to secede from the United Kingdom; shared a monarch with England, then Scotland and England, later Great Britain and later the United Kingdom from 1177 to 1949; parliamentary ties with the parliament of England and later Great Britain from 1494 to 1782; a part of the United Kingdom from 1801 to 1922; and a British Dominion from 1922 to 1937. Ireland was formerly a member of the Commonwealth, but its membership terminated when it declared itself a republic in 1949, prior to the London Declaration, which allowed republics to remain in the Commonwealth.
Israel:part of the British Mandate of Palestine until 1948. Israel's eligibility was declared in 2006 by the Commonwealth secretary-general.
Jordan:part of the British Mandate of Palestine 1920–1921; protectorate of Transjordan 1921–1946.
Kuwait: British protectorate until 1961.
Libya: In 1943 the British administered the provinces of Tripolitania and Cyrenaica until 1951.
Maldives: member of the Commonwealth until its withdrawal in 2016.
Myanmar: British Colony until 1948.
Oman: Muscat and Oman until 1971.
Qatar: British protectorate until 1971.
South Sudan: administered between 1899 and 1956, when it was a condominium of the United Kingdom and Egypt as part of Anglo-Egyptian Sudan. (South Sudan has applied to join the Commonwealth.)
Sudan: Anglo-Egyptian condominium but a British colony in reality until 1956. (Sudan has applied to join the Commonwealth.)
Suriname: English colony of Willoughbyland from 1650 to 1667 and again controlled by the British from 1799 to 1816. In 2012 Suriname expressed plans to join the Commonwealth[29] and the British government has made it a priority to provide guidance to Suriname in applying for Commonwealth membership
United Arab Emirates: seven British protectorates, known collectively as the Trucial States, until 1971.
United States
Yemen: South Yemen was a British colony (Aden) and British protectorates (Protectorate of South Arabia and the states, apart from Aden, in the Federation of South Arabia) until 1967. (Yemen has applied to join the Commonwealth.)
Zimbabwe:member of the Commonwealth until 2003.
