It depends on the building, but most parish churches in England spend between £3,000 and £8,000 a year on heating. A large medieval nave with no insulation and a gas boiler running flat out for Sunday services will sit at the top of that range. A smaller church that heats only occupied zones with radiant electric heaters can come in well under £3,000.
The biggest variable is not the fuel type but how long the system runs. Gas convection heating often needs to fire up hours before the congregation arrives just to take the chill off. Radiant systems warm people directly, so they can switch on minutes before a service and off the moment it ends. That difference in run-time is where the real savings sit.
If your church is spending more than £6,000 a year and you are unsure where it is going, ask your installer for a breakdown by zone and by hours of use. That single conversation usually reveals a quick win.