Electric panel heaters are the simplest heating option available. They mount on a wall, plug into or wire into the mains, and warm the air around them by convection. No boiler, no pipework, no flue. For a small church or a vestry that needs heating on a budget, they can be up and running in a day.
The problem is the same one that plagues any convective system in a church: hot air rises. In a building with a high ceiling, most of the warmth from a panel heater ends up well above head height within minutes. You are essentially paying to heat the roof space. In a compact room with a normal ceiling - a vestry, a meeting room, a church office - this matters much less, and panels can be perfectly adequate.
Running costs are the other sticking point. Electric panel heaters convert electricity to heat at a one-to-one ratio, and electricity costs roughly four times as much per kilowatt-hour as gas. For a building that needs heating for many hours each week, the bills add up quickly. There is no grant support specifically for panel heaters, though they do count as a net-zero compatible technology since they run on electricity that can come from renewable sources.
Panel heaters make good sense as a supplementary system - warming a vestry before a meeting, or taking the chill off a side chapel. As the primary heating for a large nave, the combination of poor heat distribution and high running costs means there are usually better options.