The law on intestacy gives everything to you.
https://www.gov.uk/inherits-someone-dies-without-will
However, under the provisions of the Inheritance (Provision for Family and Dependants) Act 1975, a person who was living in the same house as a deceased person (for two years immediately prior to the death and as if they were the spouse of that person) has the right to apply to a court for the rules on intestacy to be
varied in respect of the distribution of the deceased person's estate, in order that they can receive 'reasonable financial provision'.
https://www.legislation.gov.uk/ukpga/1975/63
Unless your mother's estate was very small, your first step should be to seek a 'grant of representation' (in the form of 'letters of administration'). Your local Probate Registry (or Sub-Registry) can provide you with the forms and help you.
https://www.lawontheweb.co.uk/personal/probate-offices
However you need to be aware of the possibility of your mother's partner seeking a court order to vary the inheritance rules. If that happens, legal bills, for both sides, can mount up very quickly; it would be better to seek an amicable agreement.