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You should double check the wording of your Lease but if it does state that you are not permitted to underlet (ie rent out), then I'm afraid the Freeholder can refuse you permission to do so, full stop.
I'm sorry you have waited a month to have confirmation from the Freeholder's Agent concerning this, but unfortunately, this delay or the matters you have done in the belief you are permitted to rent out, does not give you any special allowance.
So, legally, if the Lease does prohibit underletting, you are somewhat stuck, as your Freeholder could issue forfeiture proceedings against you (which is basically a claim to terminate the Lease) if you proceed with renting the flat out. Likewise, the Freeholder would normally write to your Mortgage Lender confirming that you are breaching the Lease.
I note that several other flats are rented out.This in itself does not make it legally OK for you to do so, but if I were you, I would contact the Agent to argue the case that other flats are being rented out and hence they should not now object to you renting out the flat. As I have said, this isn't a legal argument, but worth pursuing.
The only alternative is of course to put the Flat up for Sale. I appreciate this is not very practical, but Estate Agents can deal with the Sale even though you are abroad, and likewise, provided you have instructed a Solicitor before you leave, he can ge tthe ball rolling and do everything whilst you are abroad. The Contract and Transfer will need to be signed but this could be done by email and you returning the originals in the post.
I note there is only 68 years left on the Lease. If this does cause an issue in finding a Buyer, and the Lease needs to be extended, it would be normal procedure for you to obtain the Lease extension and complete it on the same day as you complete your Sale. This way, your Solicitor can use the Sale proceeds to pay for the Lease extension, meaning you don't have to pay for it upfront.
I am sorry this is perhaps not the answer you were looking for, but hope I have helped.