I am getting an extension built and considering getting underfloor heating fitted, would this be better than fitting a radiator?

I am assuming that the rest of your house is heated using radiators. There are benefits to using underfloor heating, It’s a very gradual and even form of heating and the absence of rads means glazing and furniture don’t need to be obstructed. However, as your boiler will be heating the remainder of your house with radiators, it will effectively be running two different systems on the one zone: one (switching on and off regularly) for the rads and one (on for a period, off for a period) for the underfloor heating. As there are two very different cycles your rad will be on much of the time and you won’t really reap the benefit of the efficiencies of either system. As a result I would advise you maintain one system (in this case radiators) for your entire ground floor heating zone. Get your plumber to size the radiator(s) correctly for the space. He will measure the cubic volume of your room and specify radiators accordingly. If its a sun room he should be able to source low height rads to avoid obstructing the glazing.