If you know your costs for gas and electricity (from your utility bill) it is a relatively straight forward calculation.
Heaters are generally rated by the amount of gas they consume. Here in the US that would be BTU/hr with 400,000 BTU/hr being pretty typical for a pool heater. Our natural gas is metered in Therms, with one Therm equal to 100,000 BTU so that same heater burns 4 Therms/hour, and here in SoCal Therms cost $2 each.
To get the heat output you need to multiply the heater efficiency (typically about 85-87%) to get the heat put into the water so using my heater as example $8 for 348k BTU = $0.023 per thousand BTU.
Heat-pumps are rated by their "COP" (Coefficient of Performance). Which is the amount of heat put out divided by the amount of power in (both in kWh). You will see numbers as high as 6 which if you read carefully is heating 80 degree water from 80 degree air, the number drops significantly if the air is colder than water you are trying to heat, and the other point they are typically rated at is 50 degree air and 80 degree water where COP is typically about 4. So in my case the electric company charges me $0.30/kWh. 1 kWh times a COP of 6 is 6 kWh of heat delivered to the water. There are 3412 BTU per kWh, so 20,472 BTU for $0.30 = $0.0146 per thousand BTU. When the COP drops to 4 then that price rises to $0.022 which is still a bit better than the natural gas cost.
Don't know if BTU (British Thermal Units) are still used in the UK or if you've gone metric for your natural gas metering, but if so that should make the calculations all the easier.
One thing to consider: The biggest heat pumps available are about 1/4 the heating capacity of the common 400kBTU/hr gas heater. That translate to needing longer to heat the pool, and quite likely meaning needing to run in the colder parts of the day where it will be less efficient.
