Placing a UTH under a ceramic tile is in direct contradiction of the instructions. I would suggest it is NOT safe as they are known to melt from thermal buildup if not allowed to be vented on one side. Enclosing them in a small space is a hazard according to all the manufacturers I know of. The pads also should be checked ever few months for damage, faults, or areas of shorting this is hard if they are sealed in, if they are not they could be exposed to spilled water and short out if wet.

I would suggest pure PVC it is heavy (thermal mass) and a ok conductor (efficient to heat through) smooth and plastic easily cleaned (grout is not so easily cleaned) It works like wood and takes mechanical fastenings well. I have not found it to sag or deform like expanded pvc does it also heats easier. I would suggest a wood top with a PVC flooring, safer than using a product in a fashion it was not intended to.