In the pool design section of this web-site we explain why we are of the opinion that heat-pumps using air as the heat source are the cheapest and most reliable way to heat the water in an outdoor swimming pool in the UK.
The best modern air source heat pumps use inverter technology to reduce running costs and will allow your to heat your outdoor pool for several hundred pounds per year - depending of couse on how hot you want the water to be and whether you have an insulating cover or not.
See Inverter + air source heat pumnps