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I will now turn to your enquiry and assist you as best i can so please bear with me.
If the loft is not demised to you under the terms of the lease and belongs to the freeholder you cannot undertake any works. If the loft is demised to you under the terms of the lease you will still require permission of the landlord to undertake any works, usually a licence in addition to any consents from the local authority. You would need to check the terms of the lease.
The difficulty is this is a leasehold property so all leases for the block will be identical in granting the same rights and obligations. You could not change the lease to suddenly demise the loft area to you.