This was thanks to the pounds 23bn road-building programme and massive hidden subsidies (the industry made no contribution to the huge environmental costs it imposed). Gordon Brown is at last doing something towards levelling the steep road/rail playing field for freight.
As to their threat to register lorries abroad: so what ? Many of the rail wagons in UK domestic traffic are registered in France and Germany. And virtually all British commercial ships fly foreign flags of convenience.
Whatever the tax regime, truckers will continue to dominate the freight industry for many years to come. If some of the corner-cutting cowboys are put out of business, that won't be a bad thing. But don't expect to see many fewer juggernauts on our roads.