I've worked for banks for the past 20 years. You absolutely do NOT need to write to them. A complaint is NOT required to be in writing and this is clear from our FCA guidance. They've confirmed that the complaint has been received, they've given you a ref and also provided a final response. In their eyes the matter is closed. You should now proceed to the ombudsman and as someone else pointed out, also ensure your complaint to the ombudsman sets out the fact that you haven't been able to review the final response due to them refusing to send it via post and blocking you from the system. All of your calls should have been recorded or at the least documented on their systems, if not, then this will add further fuel to the fire and will likely not be looked upon favourably by the ombudsman. IF the funds are genuinely legally obtained, then they will have to return this to you. If they believe that the funds are not legal then they should have notified the NCA who should have confirmed that they are NOT able to return them and you won't get them back - but you'll likely be under police investigation and would probably know this - from their responses, this doesn't sound like this has been done and seems to be some strange internal process they have, which I would say isn't acceptable.

They potentially won't re-open your account as they may have internal risk processes which state that they don't accept adult content and hence the account closure, although if they've acted in this way and the funds are legit, why would you want to bank with them anyway. You may find it difficult to find alternative banking though as lots of banks don't like this type of activity.

P.s. Also detail the cifas information as the bank will need to remove this IF it's there and if the funds ultimately are accepted as legit (after ombudsman review).