Running my Samsung HT at up to 50 deg C increases the heating cost by only 8%.
Basic battery storage will effectively result in half price electricity for heating resulting in the annual cost of space heating equal to or less than natural gas without any compromise to the homeowner other than needing to have the heating on all day.
The fundamental issue raised by the OP is that we need to avoid excessive installation cost, plant rooms and complexity when converting existing properties. (New builds would always gravitate to low temperature emitters of course but that is not the target for the great UK switchover to heat pump).
We need to remember It is not just a matter of heating efficiency but rather overall commercial effectiveness of the rollout. The primary aim is to start using renewable electricity for heating and not fossil fuels as soon as possible.