I know of some one in a similar situation.
His bank (not UB) wrote to him and pointed out that the IO period had come to an end and asked for Int and Cap payments.
He wrote back to them saying "as you know, (I love that bit), the intention of mortgage was to allow me to hold the property for resale on the expiry of the IO period. As the likely sale proceeds at the present time are less than the outstanding mortgage, his will not be possible. I await your suggestion.
He continued to pay IO and 2 years on has not heard back from them.
I certainly would not approach UB, sit back and wait. You may not hear from them for years. UB are unpredictable.