True. However, fuel suppliers are required by law to inform the relevant authorities if they suspect anything untoward. Failure to do so can result in the loss of their licence to sell fuel and criminal prosecution of responsible individuals. The companies are even required to review the fuel orders of their customers and report any unexplainable increases in orders placed for gas or low-duty diesel. Of course, I don't know how assiduous supply companies are in such matters. I haven't heard of any of them losing their licence or being prosecuted, so perhaps they are either extremely diligent or this is one of those increasing number of laws that are widely and quietly disregarded.
There is also the possibility of jealous or civic minded (self-righteous?) neighbours etc. informing the authorities.
An investigation wouldn't be pleasant. The inspectors will presume your guilt until proved innocent and the penalties are harsh. Innocence could be a little difficult to prove if your gas tank has a filler pump with a car-type fitting (they differ from commercial type fittings, in part to prevent this activity) when the inspectors arrive unannounced.
Now if your gas tank was on premises which you owned where you operate a gas-powered off-road vehicle such as a fork lift, the pump had a commercial fitting on which you used a converter for your car (widely available as they are necessary for UK vehicles which are used in Europe) and the tank wasn't visible to other people, that might make things a little difficult to prove.
Or you could just take a chance. I know that people do with low-duty (red tinted) diesel. I live in a rural area and the mechanics at my local workshop tell me that they have never worked on a farmer's car, van or truck that didn't have red diesel in the tank. Red diesel also finds its way into a large number of private vehicles; about 25% of all diesel-powered cars have red diesel in the tank when they go to the scrap yard...