If you and your partner are not married or legally united in a civil partnership, your partner will not be automatically entitled to any of the assests owned in your sole name when you die - no matter how long your relationship has been - unless you make a will.
Instead, your estate will be divided among your children (or other more distant relatives if you have no children). Even if you have no relatives, your estate will pass to the Crown rather than to your partner.
Your partner might be able to claim some of your assets if they are in need or were financially dependent on you or if you have assets in joint names. But to ensure that your partner inherits, a will is essential.