GFV vs actual equity will depend on what they can sell the car for down the line. £18.5k at 3 years old for a 5 door manual when they were a smidge over £30k seemed a little generous, more in line with likely p/x prices, £15k for the same car now seems a little stingy.
It would seem that VW have been very cautious this time around. Having bought and p/x'd 7 new VWs before my current R, i've been used to getting 85% of the prevailing dealership used sticker price at 3 years old for Golfs, Polos and Sciroccos. That usually means getting 110% of the GFV quoted.
With a GFV of £15600 the norm now and based on old patterns, i'd expect £17k in p/x. I would then expect the sticker price of a 3 year old one to be £20k. I personally think they'll fetch more than that on the dealership forecourts, more like £21-22k. The current sticker price for a 2 year old manual 5 door GTD is around £22k, and they were £25k new.
They will be going for £19k at 3 years old, and with an R always being a solid £4.5k higher list than the GTD when new, i'd say you should see at least £2.5k more for the R.
If a 3 year old R is up for £21.5k then an £18k p/x value is reasonable. For some who bought on the old terms, they will have saved themselves £90 a month but will likely have no equity in it. For someone on the new terms i'd expect them to be seeing possibly a £3k equity at trade-in.
People may mention the "glut" of leased Rs, but the roads are not swamped with them - GTDs will still likely outnumber them by 5 or 6 to 1, and that glut will have had a year to disperse by the time someone keeps one 3 years now.