I think the big question here will be your G100/G99. AC coupling means everything gets counted together. So 3kW of FIT, plus 3kW of AC coupled inverter and you are at 6kW already. If you update it all you'll need the whole lot to be G100 compliant.

I think you've got three options to look at really

1. A much bigger inverter/battery setup (and with a heatpump and agile the fact big big batteries are now affordable is useful - you really can go to 64kWh without breaking the bank so you can ride out a long spikes or charge on all the low spots. It's about 7k for 64kWh of batteries (minus 20% the installer gets back), plus whatever the costs are for a suitable inverter plus work. You just need somewhere suitable outside where they won't sink into the ground due to the weight. As you've presumably got an MCS cert for something historic you can also probably avoid the MCS overheads 8)

2. If you can't do this then a second option is to have a non grid tied second inverter/battery that just grid/solar charges but doesn't grid tie. That steps neatly outside of the G99/G100 mess but it's much trickier to then use the power effectively as you can't just throw it back into the house. Definitely a poorer option but if (like us) you had no other choices doable.

3. Insulation and getting the size of the heatpump down. Especially with air/water systems you need to be careful what order you do things. You don't want to pay for a load of pipe and radiator upgrades and a giant heatpump then render it oversized and cycling when you insulate.

Good insulation is tricky especially with older buildings but far from impossible. Mostly it's about breathable insulation, correct air flows and understanding dew points and air exchange rates. Some stuff has a terrible reputation in the UK (EWI especially) but that's often because the UK uses the cheapest usually inappropriate materials installed by idiots paid minimum wage by grifters.

In some ways the install quality is less important than you might think. A non breathable EWI (or breathable covered in non breathable cement .. sigh) will eventually fail and will eventually cause horrible damp problems that are hard to discover before real damage is done. If it's done by a cowboy it might only take 18 months, if it's done by a pro you might get 10 but the odds are not in your favour in the UK of water staying out.

A lot of insulation though isn't hard to do right - loads of insulation in the roof is easy to do right. If you have a room in roof then even with old buildings it's quite doable (sheeps wool properly treated is wonderful stuff). Underfloor insulation for old buildings needs care the airflow is as it should be and some muppet hasn't blocked the air bricks with an extension or cowboy drive layers haven't blocked it because they couldn't be bothered to dig down before laying the drive.

Plugging all the leaks is tedious but can be a fun game and is hugely effective. You can then introduce controlled ventilation with MHVR to avoid dampness (or in many cases open all the windows for 20 minutes at 7am - even in cold winter also works).

The long term cost of power is difficult to play pricing games on. Aside from the geopolitics of gas the grid is shifting with new nuclear due eventually (but likely to be HS2 grade 'soon'), and a lot of existing and new renewables that will be freed from the grid capacity mess thanks to privatisation and under-investment for decades. That's likely to make prices more volatile even if they average downwards. Export is certainly on the downward slope, and anyone planning to spend a decade doing clever battery arbitrage is increasingly going to be up against commercial battery farms doing the same thing at scale with lower costs.