In general terms you are right, except that nobody knows which way the thing will go.
IMHO, in practice you will just not be able to lay your hands on heating oil, unless you house is heated with MGO - few peeps have oil heating, and even less of them have MGO, so that's not an option.
The amount of fuel that you use in a diesel heater is very small in comparison with what you use to push the vessel through the water, and anyway manipulating reasonable quantities of that sort of fuel in cans is unpleasant and messy as well as breaking all sorts of regulations. So no point.
Apart from the pure duty / tax issue, what sticks in my craw is that your supplier is supposed to monitor your usage of the stuff. If you have a domestic installation for heating the delivery driver is supposed to monitor your tank to see if it is fitted with a draw off hose, the oil distributor monitors your usage and reports to C&E if your usage fluctuates wildly.
When buying at the marina you are supposed to give your name, rank, number and inside leg. The marina is supposed to watch you so that don't / can't transfer any to you car. If the seller doesn't toe the line then C&E can put him out of business by withdrawing his "license".
Smacks of Thought Police doesn't it. A field day for paranoiacs.....