It is not recommended that the heating system is filled with chemically softened water (i.e. typically salt based softeners).
The water used to fill a central heating system should always be kept as neutral and un-reactive as possible to protect parts such as aluminium heat exchangers within older boilers, and mild steel radiators which are common in most UK houses. To achieve this only water from a dedicated mains water feed should be used, with corrosion inhibitor chemicals typically added and tested annually.
If you have a water softener system, it should be turned off or put into ‘bypass’ mode when filling the heating system to obviate any issues. Alternatively, the filling loop feed can be connected upstream of the water softener.
The only scenario where softened water is a benefit is when it comes to a combi boiler’s hot water side (which is quite separate to the heating system). This is because softened water helps to keep the hot water heat exchanger clean, whereas calcium-laden water, when heated, can produce scale which drastically reduces heat transfer effectiveness.