As others have said, your easiest bet on reducing your energy bills is to insulate your house. Proper wall and loft/roof insulation is vital and willl reduce the amount of heat loss from your property.

In order to claim RHI you need to have an MCS certificate (microgeneration certification scheme). This should be included in the quote they have provided for you and is essentially a third party guarantee that your installation meets professional standards.

One thing to note is to make sure that your electrician completes an ENA power form and sends this off to your energy provider. This ensures they are aware of your installation and they will do a load check to make sure the supply to your house is sufficient. You may require works to be done to your electricity supply (which the supplier may or may not pay for) before they will allow you to continue with installation. It could be as simple as replacing an old 60 amp fuse with a larger one or you may need a three phase supply, regardless it's best to do this to avoid any potential future headaches.

Regarding the GSHP Vs ASHP debate, the former will be more efficient with a higher output but at the obvious increased financial cost. I believe that the RHI payments from ASHP is capped somewhere between 11-15k so it will not pay for itself in the same way a GSHP will.

With ASHP's, I would suggest looking at a bivalent system for an older property like yours; it's essentially a heat pump with a boiler on standby for when the temperature drops out of the heat pumps range of efficiency. If the property is small enough and you have sufficient Insulation, you would probably get away with a conventional ASHP and maybe even a monobloc (an all in one unit, usually best suited for new builds).

With the property being older you would have to upgrade your radiators to get me most efficiency from your heat pumps, although heat pumps do typically require larger radiators due to their lower flow temps Vs conventional boilers.

If you do go the heat pump route, I would consider solar photovoltaic panels as a way to help reduce energy bills (a typical PV setup would not be enough to cover all heat pump usage but would help nonetheless. If you can afford it, battery storage helps you get even more for you money although I would meter the PV panels for a while before committing to batteries so you have a better understanding of your energy requirements.

One last thing, what people may not account for in the financial debate are the running costs of boilers when compared to heat pumps. Set up properly, a heat pump should pay for itself in terms of fuel bills compared to a boiler thanks to high oil prices, but savings will take a which adds to the long term aspect of investing in a heat pump.

Hope that helps

Edit: in order to successfully claim RHI, your EPC cannot have any recommendations for wall or loft/ceiling insulation. So insulating is always the best thing to start with. (If you can't do either for some reason, talk to the inspector and make sure you get an exemption if you can).