The warranty specialists will, no doubt, be along soon. However as far as I am aware provided genuine parts and the correct specification lubricants are used and noted as so on the invoice any workshop can service a vehicle without affecting the vehicle warranty.
I doubt a VW Van Centre would take a TGE in, it would probably be frowned on by VW UK.
There are lots of
MAN workshops and they are often available for many more hours of the week than VW Van Centres as their primary work comes from the haulage industry where time is very precious.
My recent service on the TGE was carried out very early morning, I was on my way back home at 9:30am having arrived there at 7:00am. It took them longer to shift trucks around than do the service!