There are 2 grounds of divorce (or dissolution of a civil partnership). By far the most common ground is if you can establish that there has been an irretrievable breakdown of the marriage.
The other ground is if one of the parties to the marriage or civil partnership gets a gender recognition certificate by having a sex change formally recognised.
In order to show that your marriage has irretrievably broken down you must be able to establish one of the following facts:
This fact can only be used by married couples and not civil partners. Adultery is the voluntary sexual intercourse between a married person and a person of the opposite sex who is not the married person's spouse. This ground does not include adultery which you have condoned or connived. Condonation means that 3 months after you found out about the adultery, you are still living with your spouse as a married couple. Connivance is to have actively encouraged the adultery, such as engaging in wife swapping. The adultery has to have happened since the date of the marriage to be relevant and can include adultery after parties have separated from one another but have not yet divorced.
This is where your spouse or civil partner has behaved so badly since the date of marriage/civil partnership that you cannot be reasonably expected to continue living with them. Unreasonable behaviour is a broad term and each case is decided on its own circumstances. Some examples include financial irresponsibility, violence, ignoring emotional or sexual needs, or even nagging. The behaviour you complain about must have happened before the date of separation.
Non cohabitation for one year (with consent to divorce/dissolution)
Where you have not lived together for one year or more following your date of separation then, provided your spouse/civil partner agrees to the divorce/dissolution, a decree can be granted. Non-cohabitation also means 'living separate lives', so this might still apply if you have continued living together but have not done so as a married couple (or civil partners).
2 years' non cohabitation
The same principle applies here, but the period must be 2 years and you don't need the consent of your spouse or civil partner.
The second ground for divorce or dissolution is that you are a transsexual who has an interim gender recognition certificate. You can get an interim gender recognition certificate by applying to the Gender Recognition Panel and starting the process of getting legal recognition of your acquired gender.