I have both Velux double and Fakro triple glazed rooflights, and none showed up as a leakage problem on an airtest. The Fakro ones do not have a seperate vent, which could be a weakness in the long term (triple glazed ones only), and have an extra draughtstrip - I don't expect any airtightness problems with these.
The centre pane U-value for the triple is around 0.5, but overall roughly 1. Double glazed would only approach this as a centre pane figure, not overall, which would be 1.5+.
To minimise the edge thermal bridge, as Nick mentioned, I used the most recessed flashing kit available, which isn't the standard one, but will probably please your planning officer (the recessing means that more of the frame is within the line of the insulation).