Wasnt the Brussels -Ostend autoroute used by Jaguar for some speed record?
Yes it was also by Triumph and Sunbeam.
On May 30 1949, on the empty Ostend-Jabbeke motorway in Belgium, a prototype XK120 timed by the officials of the Royal Automobile Club of Belgium achieved an average of runs in opposing directions of 132.6 mph (ref Wiki).
Note:- Ostend-Jabbeke motorway this was the first section built. As Beagle has posted it took another 9 years to connect to Brussels.