Wet UFH sounds like the best option for your extension, this would act as a primary heating source and wouldn't need any extra source of heat either. If you're worried about the heatloss due to a lot of glass then we can close the pipe centers to 150mm to create a little more wattage per m2.
Feel free to give me a call on 01394 593305 and I'd be able to answer any further questions and prepare a quotation for you.
