I am currently building the base of a 2500 sq ft bungalow (as a contractor) and have spoken to the plumber about this as I plan to build my own house soon. We have put 150mm insulation in the floor, in 3 layers of 50mm each to overlap joints. The walls will be insulated with 60mm and the walls drylined inside.All insulation is high-density foil-backed, I used xtra-therm in the floor. I asked the plumber what he would put in his own house were he building afresh in the morning. He didn't hesitate in saying underfloor heating with solar panels and an oil condenser boiler. I'm not too well up on these things and find that the guys actually installing the systems are the best to ask. Seemingly the solar panels will heat the water to an average of 26 degrees and the boiler tops up after that. Its most expensive to heat water from 0 to 22 degrees so you're getting this done for free, having paid for the panels of course. In the house i'm currently working on, they're putting in a provision for an air/ground heat exchanger. That provision has already cost about 2 grand for the ducting alone so it'll be fairly expensive if they do decide to install it in future, I can't see it paying back. In relation to insulation, a friend completed a house recently with a high level of insulation. The windows are open more often than not to try to cool the house so his oil requirements are minimum. Spend the money on the insulation and it'll be more than half the battle.
Good luck.