You have the right to sub-let the whole or part of your property.  If you do sub-let, you become their landlord and you could be creating a tenancy.  You could have great difficulty making your sub-tenant leave if you wanted your flat back.   You could also have difficulty selling your lease if you have a "sitting tenant".  Whenever you sub let you should take legal advice to prevent such difficulties. Ask a solicitor or an advice agency how to make sure you are legally protected.

Also remember, you should ensure that your tenant does not breach the lease in any way, as any breach will be your responsibility.