Can a lack of ventilation in a roof lead to condensation in a loft?

yes it can, it may pay you to have some vents put in a roofer will show you what to put in

without actually looking at the job it is impossible to say what is causing your problems lack of proper ventalation in the roof space will always cause condensation, which in turn will rot your timbers, best roof is a cold draughty roof why has building regs not picked up on the fact there are not enought vents, ? have they signed it off ?

Lack of ventilation can cause condensation, why did he fit 5 vents, when 7 were specified. Best get him back and fit the other 2.

If the Velux window was installed with a BDX felt collar detail with transverse drainage gutter, this would solve the insulation/condensation as it not only prevents cold bridging and condensation build up, but allows any ingress of water externally to be easily deflected away from the Velux down onto the felt and into the gutter. This detail is required under part L of the Building Regulations (Robust detail) although I have encountered thousands of examples of this over the years as an ex Velux service technician. You would also need to continue the ventilation around the area where the Velux was fitted with extra vents and holes trough the rafters (See Velux best practise leaflets)