If you die without having a valid Will you are deemed to die Intestate.
When someone dies Intestate, there are rules as to who inherits the estate. It is not true that the Government automatically receive the money however it is true if there are absolutely no family members living at the date of death that the estate passes to the Crown.
If you are not married and have a long-term partner, your partner will not inherit anything. You may have children together and your estate may pass to them, possibly to be held on Trust if the children are under 18, then your partner would need to make a claim against the estate, effectively taking their own children to Court! This can be costly, thus wasting a lot of money from the estate. It would be a lot cheaper to have a Will prepared professionally.
If you are married or in a civil partnership and have children, your spouse/ civil partner does not receive all of your assets. They only receive a specified amount of money set by the government. If you have children, then some of the money may pass to them depending on the value of your estate.
If you have no spouse/civil partner or children, then your closest relatives would share your estate. They could be anywhere in the world and you may never have met them - which is probably not how you would like your estate to be distributed.