Never feel pressured to sign a tenancy agreement! It is a legally-binding contract from the moment you sign it.
Most privately rented tenancies are Assured Shorthold Tenancies. These tenancy agreements can use either very simple or very complex legal language. They are all equally legally binding.
You do not need to have a written tenancy agreement to have an Assured Shorthold Tenancy. It is possible to establish an Assured Shorthold Tenancy by simply moving in and paying the rent. You must make sure you think about it carefully before you decide to move in.