Solar water heaters cannot be used for heating, because they are open and cannot withstand pressure, while underfloor heating is closed and can withstand pressure. The two cannot therefore be combined. If those sent are flat, sealed and pressure resistant, they can be combined.
However, even if combined, they will not save much gas because the water heater generates too little heat. Even though the temperature in the water tank may reach 80 degrees at first, the temperature will drop to essentially the same temperature as the pipe in one cycle. Even if the weather is good during the day, it is not sustainable and the heating rate cannot keep up with the heat consumption. There is therefore no need to consider combining it with heating.
What is your heating surface? Is the 300 L water tank too small? How to manage excess solar hot water in summer? Have VHave you thought about restoring domestic hot water? How much solar area is planned to be used?
I suggest building a larger water tank to store solar generated hot water during the day and use it for heating at night. When the sun shines during the day, heat loss in the room is low, so a boiler can be used.