As I understand it, each dealer is allocated a certain amount of factory builds - usually on a quarterly basis.
Their quota of slots will depend on a variety of things - previous sales volumes against targets, model type and trim, etc resulting in a higher quota for high performing dealerships. It also helps smaller dealerships have (in theory), equal access to factory ordering in proportion to their bigger competitors stopping them squeezing smaller dealerships out of orders.
A high performing dealership might then go to VW UK and request extra capacity for orders, or might do VW UK a favour, take a batch of slow selling cars/quirky specification ones to clear them from VW UK stock and get some extra build slots for in demand tasty stuff as a reward.
Take a batch of ten solid brown rental spec Passat that have been stuck in a compound at the docks, and get an extra 2 build slots for a Golf R, for example
When you submit your order and the dealer accepts your order, you're not guaranteeing a production slot for your car; this slot depends on a few things, like
When in the dealer's order cycle the order is placed - the dealer may have used up its build allocations, so you will need to wait until the next quarter
The extras that have been requested - some options take longer to be ready for production than others
