Football is right. Subletting or sub-leasing in the true sense of the word, means the Landlord leases a property to Tenant A, and Tenant A, acting as Sub-Landlord then leases the entire property to Tenant B who is then a sub-tenant. In those situations, the lease usually prohibits this, or makes it subject to landlord consent.
Flatshares or flatmates on the other hand (whatever you want to call it) are only granted a license to share the property by the head tenant, which is distinct from subletting or a sub-lease. There is no requirement to enter any lease or agreement and to have the arrangement stamped for duty. Its like renting your spare room to a border. I think having an agreement is wise, but i'd want to make sure the notice period works in your favour in case they turn out to be the flatmate from hell, so having a 1 year renting period is a two-edge sword.