EuroTunnel offers (last time I checked) a special service for British lorries.

Come down to Dover with just your cab (tractor) and we will transport it to France.

Drive less than 1 km on French soil, fill up with cheap French diesel, come back and we will rail you back to England.

It does not sound like France needs cheaper diesel !

Alan

Consistently high prices are what is needed, instead of inefficiently crossing the channel for no purpose other than those created by varying tax regimes.
The Channel tunnel should be transporting more goods by rail, not lorries to fill up their tanks.