Hello I am a solicitor with over 15 years experience i will try to help you with this.
Why has the freeholder got any say in whether you lease out your flat? ( I assume that by long lease you mean you hold a leasehold interest in the flat meaning it was originally granted for 70+ years).
(a) Why has the freeholder got any financial interest in this? is ther a term in your lease which allows them to charge.
(b) Unless there is something in your lease giving the freeholder the right to impose conditions then I don't see why the terms of the tenancy you want to grant are any of their business.
So does your lease agreement give them the right to charge and to interfere.