How do I bathroom / kitchen / extension? etc.

UFH needs a screed poured over the pipes to encapsulate them. You'll need to wait weeks before tiling onto the screed. You can get quicker drying screeds for more money.

Has anyone done a heat loss calc for the room? Probably fine if it's a kitchen, but if there are three external solid walls and no insulation under the UFH pipes that room will never be warm. You may be able to mitigate some of this by reducing the UFH pipe centres to an absolute minimum. Plumber will moan but in a space that size it'll take an hour, and you only get to do it once. The joists may get in the way but a snail pattern allows for tighter spacing without 180º tight turns.