To die intestate means you have not left behind a valid Will. There are strict rules regarding what will happen to your Estate: the government dictates who should inherit your possessions, property and money through The Laws of Intestacy.
The proportion of your Estate that your spouse receives is limited with the balance held in trust for your children, potentially causing unnecessary hardship within the grieving family. If there are no children, the amount of your estate that goes to your spouse is limited and the remainder would go to any surviving parents. Should your parents be deceased, then that share will pass next to your siblings.
An unmarried partner is not entitled to ANYTHING. Here, your Estate is divided between (in order): your children, parents, siblings, grandparents, aunts and uncles. Should you have no surviving relatives, your Estate will pass to the State.
The authorities will intervene and decide who will look after your children, and where the funds for this are to come from. No provision is made for step children.
In some instances, your estranged or ex-husband/wife may have continuing rights and be eligible to claim an inheritance from your Estate, to the detriment of your new family.
40% tax must be paid over the Nil-rate band which could be mitigated in part or fully with advanced planning.