1 Radiators can get a room up to 21 degrees or so from ambient quite quickly. UFH is a low and slow approach- you are warming a lot of mass with a fairly low W/sq metre so rapid on/off control with motorised valves etc won't work all that well. You can go that road if you want to but it'll add significantly to system cost and complexity.
2 Well worth knocking on doors and asking, bear in mind that a new estate with a couple of houses completed will probably have excellent flow/pressure from a main specced for the full estate. As more demand is placed on the main the available flow and pressure will drop.
3 Unvented hot water has no header tank, the cylinder is at mains pressure. If you have good (20 litres/minute + at 2 bar +) then your shower experience will be excellent. If mains supply is at minimum (10 litres/minute at 1 bar) it'll be disappointing. Unvented (sealed) heating has no header tank either.
4 No they don't. This is where your UFH may score highly with its low and slow approach- the mass of floor being heated will take a long time to cool off. With your modern well-insulated house you won't notice the heating dropping out when hw priority chimes in, a 300 litre cylinder will take 25 kwh to heat from stone cold (15 degrees) to piping hot (65 degrees). You won't be dong that very often.