The last time I had to replace discs was because they had corroded from being so old, long before they were at risk of reaching minimum thickness.
I'm always a bit sceptical with recommendations to change pads and discs, as they're an easy money spinner and people don't want to chance it with the thing that stops the car hitting other things.
It doesn't say much for the quality or lifespan of modern VW parts if discs are hitting minimum thickness on a single set of pads in 40,000 miles.
If you've got a half decent independent garage I think it would be worth a second opinion before coughing up £400. Not kwik fit though, they'll tell you the rears are shot too and you need a new exhaust.
Edit - also be wary of how they report wear to you. They do things like saying "disc is at 25.5mm sir, the minimum thickness is 24mm". You panic because this sounds like nothing but what they don't tell you is the disc only started life at 26mm, so it's only 25% of the way through to the minimum really.