It is important to make a valid will so that the people you wish to benefit from your estate do so. In addition you could avoid beneficiaries having to pay unnecessary inheritance tax.
People who fail to make a will, will have their estate dealt with according to the intestacy rules. Where a person dies intestate and leaves a spouse but no children or other descendants, the spouse is entitled to the whole estate and personal chattels.
However the position is more complicated if there are children or other dependants. In this case the spouse inherits £250000, any personal chattels plus half of the remaining estate. The other half of the remainder is inherited by the dependants but is subject to inheritance tax which in relation to a large estate could mean a hefty inheritance tax bill.