There are numerous reasons why it's a good idea to make a will, which are summed up in my article Money tip #14 – Make a will (including why and how). Not only can a will enable you to save a lot of tax and pass on more of your assets to the next generation but more importantly you can ensure the people you care about are provided for after you've gone.
If you die without a will then it's possible that estranged relatives could inherit your wealth at the expense of those you'd rather benefit. To illustrate my point I've created a flowchart which shows what would happen to your assets if you were to die without a will. (click on the image to enlarge it)
What happens to your estate if your die today without a will?
Obviously there are slight nuances to do with half brothers and sister etc. and other non-blood relatives but the flow chart gives you a good overview of the potential outcomes. Also, if for example your siblings were entitled to part of your estate but they had died before you, then their share passes to their children.
Unmarried couples watch out!
What the chart does highlight is that couples who are not married or registered as civil partners, won't automatically get a share of their partner's estate if they die without making a a will.