+1 to 1576 above.
The biggest benefit of the Wickes ones is that they have adjustment to move the bracket towards and away from the van roof - this means you can angle them correctly so the pole will actually come out of the van.
Fit them with self-tappers into the roof arches. If you need significant extra distance from the roof, screw a piece of wood to the arch with a self-tapper then screw into the block of wood.
If there's not an arch where you need one (there isn't in mine) then you'll need to fit a piece of wood across the width of the van attached at the end to whatever bit you can self-tap into.
Vin