Hello Adam, we have 21 south facing panels (3.89kW) that were fitted 12 or 13 years ago, and they have paid for themselves a couple of times over already (we benefit from the highest FIT payment).
In February this year we installed a 13.5 kWh Tesla home battery and Gateway, and since fitting we have "saved" about £1200 from not pulling any energy from the grid. We have had an ASHP for all our heating and hot water for the last 10 years which also saves on heating costs, and now have 'his' and 'hers' EVs (MG4 and Renault Zoe), so we are an all electric household.
Since April, we have paid nothing to charge the cars which we tend to do on a granny lead when the solar panels are producing energy, but in the last week or two, the solar energy levels have dropped quite a lot and so we will soon have to start importing from the the grid during off peak periods, to get both cars and the probably the home battery charged up for daily use. As we have not yet had the battery for a whole year, we do not yet know what the total benefits will be, however, we do already know they will significant.
Although you will not be able to get the high payment FIT payments that we get, having your panels installed will eventually be money in the bank and/or 'free' miles travelled.
I have advised my grown up kids and my brother for a couple of years that if they have any money to invest, then solar panels followed by a house battery is almost a no brainer. My brother should be having his system fitted in the next few weeks.