in relation to your heating system, what ever heat pump you decide to go for you will have to be careful as the temperature of a thermal store can get up to excessivly high temperatures depending what is connected to it.
I am not a fan of thermal stores due to the fact that when no solar is avaliable you have to heat the thermal store to a high temperature which is generally costly and a waste of energy.
If you were to separate your hot water requirements from your space heating requirements you are able to sucessfully only heat up a small amount of water in an indirect coil to heat the hot water cylinder instead of heating 300 litres of thermal store to give you your hot water.
I have worked on a few thermal stores with heat pumps and what i have noticed is that the customer in all cases has had expensive electric bills due to the fact they are heating up too much hot water, same goes with buffer tanks.
A good heat pump will not require a buffer tank to be used.
you are best to connect your solar to domestic hot water and use the heat pump to do the space heating, this way you can have 2 setpoints, space heating at a lower temperature than the domestic hot water which will increase your efficiency.