We cannot speak for every system, but the electric switch LW934 (13A) would make or break the connection for hot water demand only. So if there is a timer for the hot water already in place, then our electric switch would replace this, then connect the demand for hot water or disconnect it similar to your pre-existing timer.
If you have a combi boiler then it is highly unlikely you would need an Electric Switch as the water will only be heating on a "demand" system when the water outlets are in use. The Electric Switch for Hot Water control is typically only required if you have a hot water tank which needs to be heated before any hot water is desired.
Ofcourse with our electric switch you would have the benefit of being able to check on the status and change the schedule for each day on demand using the app for control.
We are not currently familiar with underfloor heating systems and thus this is just an interpretation of how our system in theory can be linked with such a system.