For private accommodation (not halls of residence), landlords may require international students to provide a guarantor as a condition of the housing contract. The guarantor – a third-party individual or organisation – guarantees to pay the landlord any rent which the student fails to pay and the cost of any damage for which the student is responsible. Landlords generally insist that a guarantor is UK-based.
Landlords for privately-rented shared houses do not usually insist on you naming a guarantor. Landlords of large-scale purpose-built housing developments in the private sector are more likely to ask you to provide a guarantor. If you cannot provide one, they will probably ask you to pay all of the rent up front for the full contract before you move in.