Hi makessense
What is there to resolve?
You have an interpretation of your contract which differs from UB. This happens from time to time.
You discuss it with the bank informally first.
If they don't resolve it to your satisfaction, then you make a formal complaint and ask for a Final Response Letter.
When you get this, you make a formal complaint to the Ombudsman who will make the final decision.
In most cases I have seen, the letters of offer were crystal clear. The rate will be the rate operational on date of drawdown. If UB has worded their letter of offer to you incorrectly, then go to the Ombudsman as soon as possible.
On the tracker mortgages, UB reject all claims even where the Ombudsman has ruled against them on the issue in question. They force the borrower to go to the Ombudsman on the grounds that they know that many of the borrowers won't bother or won't persevere. And in some cases, the Ombudsman will just make a perverse deicision in favour of UB.
I doubt that this is happening here. I suspect that you are just interpreting the material incorrectly. But if you are right and UB is in the wrong, then go for it. Please do publish the outcome here. Because if it's wrong for you, it's a systematic error on UB's part and many others could benefit.