Assuming the storage heater is dry, clean, prepared and not subject to temperatures over 90°C we can recommend Rust-Oleum CombiColor Original.

This is available in many RAL and BS colours.

As we don’t know what actual surface temperature the storage heaters will get up to we can’t say for sure if the paint will be OK on any particular storage heater, it could depend on the model, type etc. You will need to check with the manufacturer to find out what the surface temperature of their particular heater will get to during use.

If it is below 90°C then Rust-Oleum CombiColor Original could be used.