Heating a pond doesn't mean the water has to be warm all year round. Heating a pond to stabilise temperatures through autumn, winter, and spring can be a massive benefit to koi health.
Electric element heaters (such as Elecro) are the most expensive heaters to run but the cheapest to buy. Air Source Heat Pumps offer excellent energy efficiency but the initial outlay is higher. The advantage of the air source heat pumps is the option to also cool the water, meaning you can keep temperatures stable all year round.
Kitsu Koi offer a full fitting service on all filters, heaters and other equipment.