Welcome Daz, this is a great community and full of knowledge, we look forward to your posts on the various aspects of your journey. As a start, insulate, insulate, insulate, (as much as you can in an existing house), you only buy insulation once, fuel costs are ongoing and WILL rise in price. Next is airtightness, draughts cause a lot of heat loss once you are well insulated. ASHP are now a better option than GSHP (IMO) and the jury is out on whether rainwater harvesting is worth the cost. There are many blogs on the site well worth reading and we have some very scientific minds here (that baffles me most of the time)🤔. Happy reading.