We installed tile in our entryway, bath and laundry, but did not put in heating. Regret it ever since. I think that since tile isn't easily replaced it is worth the extra expense and an added feature for resale. I was under the impression that it only takes a few minutes to warm up, but that would probably depend on the brand.

As said ... it all depends on the heat loss of your building / room. A passive house (with near-zero heat loss / with actual gain) can be "heated by a candle", and it's actually a problem in such places getting the heat from, say, cooking "out" of it (don't get an Aga if you have a passive house !). On the other end, a drafty 1890's victorian terrace with an EPC E will leak heat so badly - floors, walls, roofs, windows - that no efficient heating system replacement will get it warm. That the old cable-based electric underfloor heating "cooks" your feet is true (my parents "upgraded" their 1980's electric UfH to radiators around 20 years ago ... because of that as well as the running cost). That carbon-film-based electric underfloor heating with its nice and even heat distribution is a much more comfortable "feeling" is also correct. But if your heat loss notably exceeds the ability of of your heating system to introduce heat ... it won't get warm. "Wattage" ... is just a measure of heat loss / gain, Energy-per-time. If you loose heat at a rate of 1kW then you need to introduce heat at that rate to keep the room temperature the same. My advice is: Get a heat loss calculation done first, before you embark on installation of underfloor heating. The manufacturers are usually rather willing to help with that, to make sure what gets installed is suitable for purpose.

I doubt there's any logical reason for not being able to. The only thing I would imagine it's going to do is affect your heating. The underfloor heating would have to then travel through two types of flooring, thus, you may have to turn the heating up in general to achieve and acceptable ambient temperature. Just an opinion ...............i'm not an expert in underfloor heating - I'm sure there are professionals on here that are, so maybe they'll jump on the post?

We have got porcelain flooring throughout. I love the heating system but in hindsight would probably have kept it all the same type! Probably overboard but I can understand the benefits of both. With the U.K. weather it’s just really hard to predict and put it on hours before in anticipation of when it’s going to get cold so the overboard feels a bit. we used wundertrade and yes they pretty good! our builder installed it all though good luck! Either way you won’t go wrong but just be aware of the change in behaviour and anticipating when the heating should go on. once you e for your thermostats figured out it’s great