For most people, the part-ex price for a 1year old car won't be much more than that of a 2year old one, there just isn't the margin there for the dealership to make it happen.
Take a new GTI for £26k. Discount is common if you know to ask for one and negotiate. So you get one with a few options and maybe pay £25k for it. Not many people are going to want to pay more than £24k for a year old one that had an RRP of £28k (inc options), or pay £23k for a vanilla one, when they can wait an ever shorter time for a new one.
If sticker price is £24k, the dealer is going to want to make 20%, offering £19k -you lose £6k, there are a fair few demos about which are all competing at the same age/pricepoint.
Keep it for 2 years and you'll have probably lost an additional £3k. The dealer will have it up for £21k and be looking to give you £16/17k for it. This is all on the assumption that the popularity of the GTI doesn't strongly wane for everyone looking to get into an R.
I usually keep a car at least 2 years to spread out the initial loss with a year's normal depreciation. The R could really do with some special sports seats to set it apart, but otherwise it looks reasonable value next to the GTI.