I'm not clued up on it, but I think customs officer's have a lot of powers, they are acting on behalf of the queen. They can already go on to private land and test fuels, even if you're not there!
Fuels for use in vehicles (Excise Notice 75) before 1 April 2022 - GOV.UK
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Find out which fuel you can legally use in different types of vehicle before 1 April 2022.
3.2 Making your vehicle’s fuel available for testing
You must make your vehicle’s fuel available for testing if one of our officers asks you to. If you refuse to allow our officers to take a sample, we may seize the fuel and the vehicle. We may also impose a penalty for your failure to comply with this legal requirement.
If you obstruct, hinder or assault one of our officers, we may prosecute you.
3.3 Fuel tested in your absence
Your fuel can be tested in your absence. You will be told about the test and the result in writing.
4. Powers of HMRC officers
Our officers are allowed by law to:
examine any vehicle and any oil in or on it and to inspect, test or sample any oil in the fuel supply
require vehicle owners or anyone in charge of a vehicle to open or cause to be opened the fuel tank or other source of the fuel supply so that the fuel can be located and inspected, tested or sampled – if there is anything in the supply which might hinder this, it must be removed
require anyone in charge of a vehicle to produce any books or documents relating to the vehicle or to oil carried on it and which are carried by that person or on the vehicle
enter any premises (except private dwelling houses) and inspect, test and sample any oil on the premises, whether in a vehicle or elsewhere – in entering the premises, an officer may bring with them any vehicle used for carrying out official duties
require the occupier of the premises or the person in charge of them to facilitate the inspection, testing or sampling of oil on their premises or oil in the fuel supply of vehicles on the premises, irrespective of whether the oil or the vehicle belongs to that person or someone else – this includes attending normally unmanned premises when our officers require access to fuel
require anyone concerned with the sale, purchase or disposal of any oil to produce on demand any relevant books or documents
require you to provide satisfactory evidence that any fuel in your possession (including in your running tank) is fully UK duty paid