I want to sublet my property. Do I need to ask my freeholder for permission?

You should ensure that the lease allows you to sublet the property. There may be a pre-condition that you must obtain consent from your landlord

or some other party before subletting. This means that you cannot sublet without first obtaining consent. Such consent should not be unreasonably withheld. If your lease does not allow you to sublet you will need to ask your landlord if they are willing to waive this restriction.