Norbreck: If you don't cancel, I would squeeze them for every bit of goodwill they could possibly throw at you whilst also taking advantage of the fact that the p/x price of your current cars are fixed. You should be looking for a healthy additional discount such as getting some of your options free. 5% discount on the whole car is a reasonable place to compromise, ask for 10% maybe you'll get it - shy bairns get nowt.
Really put the guilt trip on them - you are buying 2 cars, that's 2 missed sales if you pull out, you've bought VWs before - really put out the VW brand loyalty. If anything non-warrantiable needs doing between what was your expected build date and pick-up time of the car (such as brakes/tyres/service) then VW UK/dealership should also be picking up the tab as it would not have been your cost to bear had you gotten a timely order fulfillment.
I can see VW UK or your dealer maybe bullsh!tting you further by giving you a non-commital attempt to bring things forward and keep you hanging on the hope they'll come througgh for you. Be firm and insist on promises not "we hope to".
Good luck with that. When VW lost my original GTD order back in July 2010 (originally ordered at the end of February), I kicked up a fuss, involving VW UK, my dealer (in the form of the dealer principal and the sales manager) and the parent company of the dealer (the company secretary). All of them got long, detailed letters setting out exactly what happened since the day I ordered. It took them a while to confirm that the order had gone AWOL and apparently it went "to the very top" of VW as they attempted to sort things out. I then got word that they had to re-order the car (after they'd actually done so). So I waited for another four months (the original time that the salesman estimated) and finally got my car. I got the service pack for free and they let me choose up to a certain sum from the accessories catalogue. They managed the service pack fine but forgot half of the accessories I'd asked for and when I tried to buy a set of steel wheels for winter, they said they'd give me them for free. They didn't manage that either, and they suggested that 15" ones would fit when all the information I'd found suggested they wouldn't. Eight months and one day after ordering, I finally took delivery.
What you will also find is that you may get offered a courtesy car after a certain period (16 weeks, I think). I ended up running around in a basic Polo for two months. I was on the dealer's insurance, do I was able to cancel my own and get some money back. So I wouldn't worry about out-of-warranty work on your own car, unless you really want to keep it (I hated mine, so didn't).
So yes, complain, but I wouldn't expect to get anywhere at all. The dealer will wash their hands and say it's nothing to do with them. VW will fob you off saying it's issues in the factory and it'll settle down in due course and please be patient. I assume that they build the cars in batches, depending on what options have been specified; I'm fairly sure that (at least in the days of the Mark 6) they build GTDs and GTIs in batches as well, which adds to the delay.
The only problem with cancelling is that if you really, really want the car, you'll have to wait for it, possibly longer than week 40/41, if you go somewhere else. You also have to wave goodbye to any loss in value of your current car between you ordering and the point at which you cancel, as if you re-order elsewhere, you'll get less for your P/X. I seriously considered cancelling, and was told I would get my deposit back in full, but elected to continue as there wasn't a single car I could think of that I wanted more than a Golf.
I started off with a letter and got invited to my dealer to talk to the dealer principal face to face. Find out their name (I called my dealer and asked) and send them a well-drafted letter (your posts on this forum will provide a sequence of events you can relate). My strategy was to kick up the maximum amount of fuss by involving as many people as I could.