If a person dies intestate (without leaving a Will), the distribution of their assets is determined by legislation such as the Administration Act 1969 and the Property (Relationships) Act 1976, and may be distributed differently to the way the person had envisioned it would be.
A marriage automatically invalidates all Wills that were made prior to the date of the marriage, unless the Will expressly states that it has been made in contemplation of the marriage.
A review of any other existing estate planning arrangements (such as existing family trusts, memorandum of wishes, powers of attorney, insurance policies, etc) should also be undertaken prior to marriage, as they too may no longer be relevant or appropriate after the marriage.