Any time from March 13 will see the first same sex marriage in Britain. As you may know, the UK has had civil partnerships since 2005 however since then the power to legislate on this area has been devolved to Scotland and Northern Ireland which have separate legal systems.
Equal marriage legislation has been passed at Westminster covering England and Wales and at Holyrood with the Scottish Parliament passing parallel legislation. No new laws have been proposed for Northern Ireland however marriages in the other constituent countries of the UK will be recognized there.
On March 13, those married under foreign law will have full recognition as being married. The date is also the first when couples can "give notice of their intention to marry". The normal procedure is for the couple to give 15 days notice after which the ceremony can take place. The uncertainty over when the first will be is because of provisions for the waiting period can be waived in special circumstances. The government web site outlining the provisions has an example some will find interesting.
The Registrar General can allow a marriage to take place without the normal 15-day notice period where one of the couple is seriously ill and not expected to recover. Such marriages of same sex couples will be possible from Thursday 13 March 2014. The Registrar General can also reduce the normal 15-day waiting period in compelling circumstances, which may include where a person is being deployed overseas in the armed forces to a war zone before the usual 15 day period is completed.
The waiting period was waived when the first civil partnership took place. In that case one of the men was terminally ill.
Certain provisions of the law will not come into force on that day:
Marriages of same sex couples in some British consulates and armed forces bases overseas will be possible, and arrangements for marriages of same sex couples in military chapels will be in place, from 3 June 2014.
There are complications surrounding those who have previously been married or entered into a civil partnership and who subsequently received a Gender Recognition Certificate. These are given under the Gender Recognition Act and would normally follow re-assignment surgery. In effect, the previous marriage or civil partnership was automatically dissolved as the former could only be entered into by different sex couples and the latter by same sex couples. Under the new provisions, civil partnerships are still not available to different sex couples (the legal provisions including the dissolution of the partnership rather than divorce are different) but previous civil partnerships can be converted to marriages without further ceremony. The Regulations regarding the continuing of these partnerships and marriages are still being drafted (you can imagine the complexity of trying to accommodate a variety of different wishes the couples might have). The government expects these Regulations to be in force before the end of the year.
Ongoing are reviews required by the legislation. These include the operation of the Civil Partnership provisions (some mixed-sex couples have been pushing for partnerships to be extended to them) and the different provisions for same sex and mixed sex couples within various occupational pension schemes. (My own local government pension provisions for example allow for survivor's benefits to be paid but only take into account contributions from a certain date when calculating the benefit rather than for widows or female survivors whose benefits would be calculated for the whole contributing period. In the past an unmarried survivor had to show dependency to get a "widow's" pension.)
Religious bodies have exemption from the normal marriage provisions. While most may conduct them, the Church of England would require separate legislation to be passed by its General Synod and then Parliament. Permission for others is written into the Act but Humanists do not fall withing the "faith organisations" provisions and have asked that their celebrants be given this right.
Note there are other difference between the "ordinary" and same-sex marriages. The latter cannot be annulled on the grounds of non-consummation and adultery is not a ground for divorce (and anyway is now a reason to demonstrate the "irretrievable breakdown of marriage" which is the main legal "reason" for divorce.) One omission in the Act is the failure to extend to same sex marriages the provisions in the Human Fertilization and Embryology Act 2008 under which a child born to a woman in a civil partnership is presumed to be the child of both partners. As it currently stands, the birth certificate can only show the name of the woman who gave birth. Rationalizing that would appear to require primary legislation.
(Quotes from UK government web sites posted as fair use and under the Open Government Licence. References to "same sex marriage" reflect the title of the Act.)