Broadgage mentions a recent change to the arrangements on red diesel, which seriously affects generators for commercial use. The notice is here:

www.gov.uk/guidance/using-rebated-fuels-in-vehicles-and-machines-excise-notice-75-from-1-april-2022#when-you-can-use-rebated-diesel

but is fairly unhelpful in this case. There are some options and exceptions and you can apply to have the condition removed, and this would seem reasonable where there is no alternative mains supply. The change in the regulations is in my view highly unreasonable, as fuel duties for road vehicles was the previous definition. Bringing in almost all other plant and machinery with IC engines (including boats) seems to be another tax grab, or perhaps something to do with the "Green" agenda, but there is of course no other option to most portable generators, site plant and vehicles and boats of any size. I would expect the changed to be challenged at some point by a group of fossil fuel users, I assume the government thinks electric 1000 HP Bulldozers are available, along with 50 acres of solar panels to charge it, but then it could do very little work as they normally operate 24/7 and this change would cost the operator about £3,000 per day!

Back to the OP. The EFLI of your design is nothing to do with the Earth electrode resistance, and excessive effort to get this very low is unnecessary. I suggest Foundation Earthing should be examined as the most economical for a new building, which is obviously of significant size. Earth Fault Loop Impedance in this context is simply the resistance between the live supply cable and the Earth wiring in the building, which will vary widely with circuit size and length. It is simply the number we use to ensure that an Earth fault at a point of utilization in the installation will trip the circuit protective device with a suitable time-span, typically 0.4 seconds for final circuits and 5 seconds for distribution circuits. It therefore has nothing to do with Electrode resistance. The Earth resistance to true Earth may well be a number of Ohms, and within reason is of no consequence, 5-10 Ohms is fine and easily obtained with foundation Earthing because these are usually wet and of very significant surface area. The Earth connection is a solid cable to all the reinforcement, this all being connected usually by welding.

So far as I can see, it could be covered under:-

2.4 Heating and electricity

Rebated diesel may be used for non-commercial premises for:

• heating
• generating electricity

So far as I can see, it could be covered under:-

2.4 Heating and electricity

Rebated diesel may be used for non-commercial premises for:

• heating
• generating electricity