My experience of these things is
Persist, and state the facts in each communication. In full, adding each new communications to the ever extending thread, copying and pasting there inevitable responses into the thread to make one coherent document, that if read by a third party would provide the whole story, questions and responses from all parties.
State the facts, backed by your legal rights, legislation/ law etc.
Let the dealer know unequivocally that you are not going to roll over.
Let the dealer know what you will / will not settle for, stating what you expect the outcome to achieve.
I am sure there will be much prevarication from the dealer, they will try to take the argument in various directions, to muddy the issues.
The bottom line is this, the van had a several faults from new which you highlighted at the first opportunity, an opportunity which was cut short when the initial fault identified by you prevented you from taking “ownership” and therefore “proper inspection “ of the vehicle until the initial repair was carried out, at which stage you inspected the vehicle and advised of the other paint issue, which subsequently became a larger issues, regardless of the original extent of paint issues raised by yourself, which was highlighted as an additional issue to the first, subsequently you advised the dealer of further paint issues on other panels, pertaining to the first issue reported & in addition to the two faults already reported.
The Dealer is not able to repair multiple faulty panels to “Factory specification”, it is merely a remedial action, which will impact the value of your vehicle and potential resale value. Due to the serious paint issues highlighted by yourself, which are manufacturing faults, and are acknowledged by VW U.K. as faults, your “new” vehicle is considered faulty by the very nature of the various and numerous faults. (not just limited to paint). The potential to repair the roof may cause issues with a know problem with corrosion of the “California” roof, which is a well documented fault which is recognised by VW gmbh, this later point would mean that you reject the vehicle on the basis that the paint has not been applied correctly at the factory to various panels including the roof which is known to suffer from bi metallic corrosion due to incorrect paint application, and remedial measures offered may cause longer term issues, further VW GmbH has provided several remedial measures to the known paint issues (roof) to prevent / reduce the incidence of the known issue, the fact the paint has not been applied incorrectly to various panels and that VW U.K. have specifically advised you to reject the vehicle due to multiple faults and most notably multi paint faults on multiple panels including the roof leads you to the conclusion that the vehicle should have been rejected at the factory, although it appears that the issues were not identified at the factory and the vehicle was shipped to the dealer who, during their preparation failed to notice any of the reported faults, despite the dealer check involving 140+ Quality checks ! I exercise my right to reject the faulty vehicle due to the amount one extent of paint issues and the potential issues the proposed remedial actions may have in future and particularly petaling to the know issues with roof corrosion.
I have lost X amount of days whilst the initial reported fault was fixed, then another paint defect was reported and investigated, I did not have ownership of the vehicle to carry out proposer inspection, therefore I was unable to take accept ownership of the vehicle and proper extensive inspection, the fact that subsequently additional paint issues were highlighted further raises the issue of the quality checks from factory and dealer to identify quality issues.
I hope this helps, I could go on a lot longer but I think I make the point and tone you need to follow to succeed.
I know it’s not right but the dealer must fob you off or loose money ! It’s your job to be tenacious and point out the facts succinctly backed by fact and reason.
Incidentally is the dealer part of an ombudsman scheme ? They will not like the threat of that ! It will cost them money (not you) to go through an ombudsman who will probably rule in your favour.
I have no doubt the dealer will offer you a refund, make sure it is a 100% refunds with all costs reimbursed.
If so take the money and run, if not make yourself a nuisance on a daily basis and escalate within the business