Help with changes to current set up

Ectophile

Do check with your DNO (District Network Operator) before installing a larger solar inverter.  The existing system may have been capped at 3.6kW for a good reason.  3.6kW is the maximum that usually goes through without any surveys.  More than that would require prior approval.

Before spending thousands on batteries, work out if the cost savings - the difference between day rate and night rate - will ever pay back the cost of the battery.

Also make sure you aren't going to overload your supply.  Charging an EV, and running the heat pump, and charging the house batteries, and running the immersion heater all at the same time may require an excessive amount of power.