Depends on what you mean by "UTH". You shouldn't even attempt it with the common UTH. It can be accomplished with flexwatt or other brand heat tape though. Usually you'd want to rout out the section of the floor where the flexwatt will be. Then after you get the flexwatt in and setup, you'd want to cover it with something like foil. Alternatively you could bypass the routing and just set the flexwatt up then cover it with either a piece of plexiglass or thin ceramic tile.
Most of those options are a lot more complicated than just going and getting an appropriately sized Radiant Heat Panel, which is sort of like a UTH for the top of the enclosure instead of the bottom. They're simple to install as long as the wooden viv has a wooden top too.