Somehow I suspect that you missed another atvantage of the trailer as it is there.
We talked about your project a little in the off-topic posts, in the post about a book featuring Reliant Robin.
Tom linked to your post.
We know that A2's tank is actually tiny. I (myself) carry a 20l jerry can in my rear that I even made a fitment for and have a pump to extract the diesel from it just to minimize gas station drive-ins and enjoy the advantage if I find cheaper fuel.
It helped significantly when I drove to Germany through Poland. Polish fuel is almost ⅓ cheaper than german and the can let me avoid refuelling there.
The trailer has a fuel tank as it is originally in the rear of the car.
One thing you can add to the trailer to actually use it, is a hose from the trailer's fuel pump with a removable (I stupidly forgot the exact word to describe it) coupling near the coupling of the trailer itself, to either your fuel tank or to your fuel line via a Y joint. There are hose couplings that seal up when uncoupled.
A remote controlled on/off switch may be used to activate the fuel pump in the trailer. Or with just a couple of additional wires for the on-off switch and a fuel level sender for the trailer tank you could simplify the task and add an idea about the situation in there.
I know that the gas stations in Britain are in rather close proximity to each other but the prices of the fuel probably vary.
At least we have a network of fuel stations where the price is more than 10¢ (that's for a liter, not a gallon) lower than in the next cheapest network and those stations are quite sparse and located only on the west side of the country. Another thing we know that a trailer adds resistance to the car's rolling and by that it reduces the range with its tank.