In the first instance, I'd complian to IIB. Look for a manager, rather than an agent. They should have a record of giving you approval in pronciple for >80% in September. See if you can get them to change their mind as they were the ones who evidently made the mistake.
If that doesn't work, consider re-mortgaging. Given that the money is for home improvement, you should be able to get 90% elsewhere if your credit rating is good and you have the income to support the application.