Thanks Simon,
It's certainly odd that Gevaert appear to have been operating throughout the War in London, but then as you say the British Government were involved in overseeing photographic production at that time. Ilford themselves under the Air Ministry, presumably Barnet Ensign as well. Gevaert would not have escaped some form of Government control.
Focal Press began publishing their Focal Guides for a wide variety of cameras during the war at the behest of the Government because there was as shortage/abscence of Instruction manauls for many of the cameras donated to help the war effort. In addition they published Kurt Jacobson's Devoping and also Enlarging. Three of the key people involved with Focal Press, Andor Kraszna-Krausz ,Kurt (Curt) Jacobson and LA Mannheim, were all jewish emigres who'd left Germany.
Gevaert was based in Antwerp a city with a large Jewish population, presumably some employees had left Belgium for London before Geramny invaded. So the Government must have found some use for the London distribution centre.
For Simon

Ilford always used to have the last advert
Ian