Several grant schemes can help. The Boiler Upgrade Scheme offers £7,500 towards a heat pump installation. The National Churches Trust runs several grant programmes for building improvements including heating. Many dioceses have their own funds, and some are specifically earmarked for decarbonisation work.
The Listed Places of Worship Grant Scheme refunds VAT on eligible repair and maintenance work, which can include parts of a heating installation. The Places of Worship Renewal Fund and Heritage Lottery Fund cover larger projects where heating is part of a wider scheme.
Most grants require you to apply before the work starts, and some need matched funding. Start early: a typical grant application takes two to six months from submission to decision.