I would pursue it in the hope that it might work!
Realistically UB can take a name off no problem, however of course they don't want to as if they say no then you may end up having to take out a new mortgage and then lose the tracker, see how they might prefer that!
Have you tried the mortgage advisor in a local branch? Is your income going to be sufficient for the loan, remember they will stress test too so it's not a case of being able to pay the existing payments. I would concentrate on getting his name off first, it that is successful then you can add husband's name but that is not even a necessity. But if you need husbands income to qualify for the mortgage at the moment then you are in trouble because they will definitely insist on a new mortgage then.
Try and get them to underwrite it based on your income and value, then if they say your income is insufficient you can offer to pay off a chunk and see if that brings you within the guidelines, I wouldn't mention that upfront until you see what their attitude is.