It's my property so I'll let it to whom I want!
This might be the understandable sentiment of many a landlord but with modern laws the landlord must not discriminate on certain specified grounds:
Landlords and letting agents should familiarise themselves with the legislation in this area and should also be aware of the requirements of the Human Rights Act
However, the modern landlord is able to discriminate on those critical factors which qualify or disqualify tenants, for example:
So long as you don't discriminate on the specified grounds you can let to whoever you like. If you don't want smokers, tenants with children, tenants with pets, etc, you don't have to take them.
It is important to bear in mind that landlords should be consistent in the way they apply their tenant selection criteria. For example, it would be discriminatory to put some tenants through credit check procedures and not do this for others.
In the event of legal proceeding taken against a landlord for discrimination it is vital that documentary evidence is available to verify the selection procedure and the criteria applied.