MrSavage: is it really £11k lost though? Was nigh on half of the OPs £8k deposit actually discount (I suspect so as you can't PCP less than 70% of RRP, and i'd have expected him to have gotten some discount on the GTI) ? If so it's not money lost. On my proposed figures he'd "only" be about £8k down after 18 months, which is pretty much as expected for changing at 18 months. It's a lot of money, but so's depreciation on any £25-30k. If GFVs don't soften, he'll make a fair bit of that back in considerably better residuals which will pay for the extra fuelling costs many times over. The actual fuelling costs of R will most likely be about £45 a month, noticeable, but not exactly a fortune. If he's wanting to jump on the lease list then perhaps better to do it sooner rather than later.
On the other hand, it seems that the PP is a great enhancement to the GTI, the OP is already in a great car, he'd only suffer an extra £4k depreciation keeping it for 3 years. It definitely makes more sense to lease than PCP (i'm buying mine and skipping PCP interest, it's just about worth buying over leasing on those terms) an R that isn't packed with extras. I do think the OP has far less to gain in the upgrade than the likes of myself with a slippery Bridgestone shod GTD with a real want for more grip, more power and not caring so much my mpg is going to get hammered. If I had a PP already, it'd be a tougher decision for sure.