Heating is a church's single biggest carbon emitter, by far. If you do nothing else, replacing a fossil fuel heating system with an all-electric alternative is the single largest step your church can take towards net zero. The Church of England has committed to net zero carbon by 2030, and heating is where that target will be won or lost for most parishes.
Start by understanding what you use now. Get your gas and electricity bills for the last two years, work out your annual consumption in kilowatt-hours, and convert that to carbon using the figures on the government's conversion factor tables. That gives you a baseline. Then look at what you can change: switching from gas to electric heating, adding controls so you heat only when the building is in use, and exploring renewable generation such as solar PV on a church hall roof.
You do not have to do everything at once. A phased plan that starts with the biggest win (usually heating) and adds improvements over time is more realistic than a single capital project that the PCC cannot afford. Talk to your diocese about their net zero support programme. Most now have a dedicated officer or working group who can help you plan the route.