Be interested to know which one is OK. The P38 meets Euro 3 so shouldn't be ULEZ compliant but there is obviously an error in the TfL database as it shows a 2000 or later, 4.0 litre P38 as complaint but a 4.6 isn't, even though they both meet the same standard. One other loophole was filled when they changed from the T-Charge to ULEZ. Under T-Charge an import, which doesn't have emissions data on the V5, was considered exempt, whereas under ULEZ, it isn't.
Found the reply I was looking for in amongst the 16k emails in my inbox, I've put the bit about gas in bold from their response below. Not in response to a P38 (not even a Land Rover in this case) but the gas bit might be of interest. Though the only gas-only vehicle I know of is Robert's Medusa from the other forum.
"Thank you for your enquiry received on 4/3/2021. Your case reference number is *.
We appreciate that you have taken the time to contact us in relation to this matter.?
It may help if I explain that the Clean Air Zone scheme is not linked to Transport for London's Ultra-Low Emission Zone and can only be found in locations outside London. However, we are working with Transport for London to ensure as much consistency as possible between our Clean Air Zone vehicle checker and the ULEZ system. We would advise all drivers to use the relevant vehicle checker for the zone they intend to travel into if they are unsure if their vehicle meets the standards.
A vehicle would need to run solely on gas to be exempt from charges. As you have advised that the vehicle runs on both petrol and LPG gas, the vehicle does not fall under the exempt fuel type and so would still be subject to Clean Air Zone charges where that vehicle type is chargeable.
The vehicle would need to meet the minimum emission standards for the zone the vehicle will travel into or within.
If the vehicle does meet the required emission standards, it would not be subject to charges.
After checking the vehicle record, I can see the Euro Status is missing.
Work is ongoing to provide an accurate result for vehicles which may have adopted a Euro standard before it was legally required to do so. You are not required to take any action at present as we are working closely with manufacturers and partners to update the results for these vehicles before the launch of the first Clean Air Zones.
If your vehicle has adopted the Euro 4 status, I would expect that it would be compliant with the emission standards of Clean Air Zones, and so would not be subject to Clean Air Zone charges.
When this information is updated, the Clean Air Zone checker will be able to provide the information to show if your vehicle is compliant or whether you would be subject to a charge when entering the Clean Air Zone.
Please check back closer to the time of the Clean Air Zone launch for this information.
I trust this is of assistance to you."
I did post something along the lines of what Davew has said earlier in the thread, but looks like in the process of "posting" it, i didn't actually press the reply button.