what about informing PVW, Golf+, EVO, Top Gear, edition 38, club GTi, newspapers, 5th gear - basically anyone in the motoring world with some sort of clout - or to sufficiently embarass VW - we all bought a car that was sold to us all as a 1500 only production run ONLY - not god knows how many more !!
Is there any actual written evidence of the production run being limited to just 1500 cars. If there is, and it is verifiable, then all 2300 Ed30 could actually sue Volkswagen for breach of contract - either as individuals, or as a group action. Trading Standards - preferably the Trading Standards for Milton Keynes council (because that is where VW UK is officially based) would definately offer some advice, and may even take up your case. It is basically selling goods under "misrepresentation".
How's this? 

Providing that is actually genuine, then that would be deemed perfectly acceptable proof that the Ed30 was being sold in just 1500 units. For VW to subsequently make, and sell more than 1500 units is deemable as a "breach of contract". This could be remedied in a variety of ways, though financial compensation would be the most reasonable at this stage.
What is also clear is that they state the car will NOT be numbered. Therefore, if VW fail to supply the plaques, then this is a more difficult legal scenario. As that letter stands, they are under no legal obligation to provide plaques. However, if they have subsequently written to Ed30 owners stating they will now issue plaques, then they will have to honour that pledge.
Whatever happens though, these plaques should simply be seen as a "goodwill guesture", and should NOT be used as a levering point for compensation for the overbuild.