I think some of the best paybacks for little investment are the passive ones. Look to improve the insulation of the property first, especially if you are gutting a building and starting off with a bare shell or doing a “Grand Designs”. Get this right and you are then needing less energy in the first place to keep the building warm.
If there is mains gas available, then most people will take that option, a condensing boiler and where possible size the heat emitters so that they work well on lower flow temperatures, so the condensing boiler actually condenses and has a chance of being 90+% efficient! Water should return to the boiler at around 54C or less to stand a chance of getting the higher quoted efficiencies.
If there is no mains gas pipe nearby or you really want to go green then a heat pump would be a good option on a well insulated building. If you’ve got the land then a Ground Source machine is generally little more efficient (but the install costs are higher), but if you don’t have the land or are on a tigher budget, then an Air Source will do just as well. Heat pumps work more efficiently at lower flow temperatures, so if you can install underfloor heating (which typically runs on lower flow temps), it will be the most efficient use of the heat pump.
A heat pump can also heat a Domestic Hot Water cylinder too. This can be in addition to a PV optimiser, dumping excess PV energy into the tank rather than exporting that PV generated energy. Some cylinders have dual coils to connect solar thermal into the tank as well, but in my opinion, a good sized PV array and a solar PV optimiser / diverter is probably more flexible and slightly cheaper.
If you want solar PV, you might want to act quickly if you can as the Feed In Tariff currently available for a new install is only available until 31st March 2019. Installs registered after this date will not be able to claim the kick backs.
Regarding PV, I’m currently looking into it too for my own home and have yet to make my mind up what would be best. Given I have electrical qualifications but not MCS, I will probably do a DIY install to minimise cost, but not claim the FIT. If a roof is partially shaded at times, then a micro-inverter setup will be more flexible and efficient but may cost more to purchase.
I’m sure others who have more PV experience than myself will offer their thoughts on micro inverters vs two larger inverters for east-west setup. What I would say is, if you are going to have PV you should look at maximising your use of the PV energy, either via a PV optimiser (channelling excess energy into a hot water cylinder) or consider battery storage / Electric Vehicle charging when there is sufficient PV energy to minimise grid usage. The OpenEVSE project is also on my radar, as is a Renault Zoe one day!
Lastly, if you are radically improving the insulation levels of the property, this usually means reducing the amount of natural air movement / air changes within the building. Have a look at MVHR - Mechanical Ventilation Heat Recovery. Typically this will consist of a central fan unit with an air heat exchanger and duct work around the building to ensure frequent air changes whilst pre-heating the incomming air using the heat from the exhaust air. Too many new builds are well insulated, but full of stale stuffy air. I’m sure that’s given you plenty to think about