Hi Plumbob
Yes, underfloor heating is very popular, but just bear in mind that you are very isolated to a specific area of plumbing and not every body will want this work done. Therefore you may have quiet days from time to time.
To fill the quiet gap, you should be doing other plumbing work.
Look here for plumbing & tiling courses:
Plumbing Courses from Chase Plumbing Academy
Tiling Courses and Wall and Flooring NVQ Courses by Chase Tiling Academy
Good luck!!