The National Lottery Community Fund is the largest funder of community activity in the UK. It is not a church funder, but churches that serve their communities regularly receive grants through its programmes.
The most accessible route is Awards for All, which offers grants of £300 to £10,000 for projects that support communities. A church heating improvement could qualify if the building is used regularly for community activities and the application focuses on the people who benefit, not just the building fabric. The fund has supported church heating projects in the past under this framing.
For larger heating projects, the fund's main programmes (Reaching Communities in England, or equivalent programmes in Scotland, Wales and Northern Ireland) offer grants up to £500,000 over five years. These are more competitive and require a strong case for community impact, but a church that is a genuine community hub with regular non-worship use could make a credible application.
The key principle across all National Lottery Community Fund programmes is that the project must be about people and communities, not buildings. Frame your application around who benefits from a warm, usable building, not around the heating system itself.