Do you really know the definition of low level? It's below 2000ft AGL.
There is no reason why a route callsign could not depart at low level at all.
A departure from Northolt via a VFR route or corridor was also standard practice - no reason it would change now.
Northolt to Leeming via airways would probably be a very scenic route so VFR or military radar is the preferred option.
Does that clear it up?
