There are two basic reasons for this. The first is that if a pool is covered with a floating insulated cover, when not in use, an air source heat pump will reduce the cost of heating a pool to a few hundred pounds a year. Covers are now regarded as essential for safety purposes and so they effectively kill two birds with one stone!
The second reason is one that very few people are aware of. If the pool water is circulated 24/7 by a carefully sized pool pump the pool water can be disinfected using an ultra violet light cell. This allows the pool water chlorine levels to be reduced down to the same approximate level as the water that comes out of your kitchen tap. This does rely on a modern accurate pH and chlorine measurement and dosing system but there are two benefits - the chlorine water levels are minimised to tap water level and the amount of chemicals required is substantially reduced.
On this basis, the cost of running an outdoor 10m x 5m pool could be as low as £400 depending on the length of the swimming season and the pool water temperature settings.
Indoor pools are always going to cost more to run than an outdoor pool - mainly because they are used all year round.
However, the factors which reduce the running costs of Outdoor pools are the same for Indoor pools.
Furthermore, modern air handling units that are now required by the Building Regulations provide the means to absolutely minimise the heat requirements of an indoor swimming pool.
These units include:
Air Handling Units provide an excellent pool hall environment and will only cost about £2000 per year to operate in a pool hall containg a 10m x 5m swimming pool.