Sometimes, but not always. Most churches built or rewired in the last forty years have a three-phase electricity supply, which can handle a reasonable heating load. Older single-phase supplies may need upgrading, which means contacting your distribution network operator and potentially paying for a new connection.
The internal wiring is a separate question. Even if the main supply is adequate, the existing circuits may not have the capacity for additional heating loads. An electrician will need to assess the distribution board, cable sizes and circuit protection. In some cases the existing board can accommodate new circuits; in others it needs replacing.
Budget for an electrical survey early in the process. A supply upgrade, if needed, can take several months to arrange and adds several thousand pounds to the project cost.
Where an upgrade is not practical, battery storage can work around a limited supply. See power supply and battery storage.