The answer to this question will depend on the situation. A landlord cannot refuse to consent to sublet or assignent without providing written reasons for the refusal.
Some examples of when the landlord may refuse to consent to a sublet or assignment is if they require the prospective subtenant to fill out an application form and the subtenant does not do so or refuses to do so. The landlord could also refuse to consent if the prospective subtenant does not have enough income to pay rent.
January 2015