Ask the professionals as the insurance should accept the modification if it has been installed by a professional company. If it's a DIY job then they may be able to point you in the right direction in order to be able to cross the barrier regarding insurance.
Many insurers will accept windows being fitted into vans. They may charge extra for cover. Shop around for a price to fit the glass as there is a big variation in cost. You can usually get "pattern" glass ( in your case non-vauxhall) which can be cheaper. My brother had trouble getting his insurance to cover his van for just fitting alloy wheels. One company said that the wheels had to be exactly the same size as original fitment wheels from the manufacturer, in his case Nissan. You have to be prepared to swap insurers and pay for it though try Shield
------------- cramming for my finals in the twilight zone
If it has factory fitted rear seats like an estate car then see if you can get windows fitted & get it re-registered as a car perhaps? A Combo with seats & windows was always available as a car in other Euro countries. For example Berlingo sold in UK with windows & seats is a car & a van without, so worth investigating.
my son had a window fitted to his Renault van (same as the movano different badge) he had his done by the Renault garage and a passenger bench seat behind his and front passenger, as far as know he does not pay any extra insurance on the van, ie. it is his works van