"The devil is in the detail". The L/O is composed of the specifics of the borrowers mortgage (generally a short enough doc. as per your comment) and the general T&C's attachment. There are 2 issues of concern to you;
(1) Is there a clause in the General T&C's which can lead to a rate increase if the property is rented out?
Ask UB for a copy of this doc. You are entitled to it. Remember that it is the First Active LO that applies in your case.
(2) If there is a restrictive clause, does this mean that the Bank will be watchning for such an occasion and will impose it?
Difficult to be definitive on this. From my own experience, most of the Banks (if not all) are fully focussed on the non compliant element of their book i.e. those that are not meeting their loan repayments. I have not seen or heard of any concentrated effort by Banks to look for those who are renting out property and imposing a rate increase.
In my own view, the risk of losing the tracker is not high. There is little fear of any retrospective penalty being applied in relation to this. However, if the clause is in your loan agreement, there is the possibility of the Bank imposing a rate increase, if they become aware that the property is no longer your PDH.