Of course the base price has risen in the last 2 years, we all know that!
But as SRGTD pointed out above, you can still get a well spec’d GTi with more power and better tech for a little over £28k.
You're possibly missing the point. You are talking as if £28-29k is still cheap, that's dramatically more than what was available just a few years ago. Incremental price rises, model dropping, and other factors such as inflation and currency adjustments have caused the car to be more expensive.
I'd hardly call a car with just a pan roof and metallic paint "well spec'd" either. If you're happy with base spec then yes, but as a Golf caters for all tastes and needs, it soon becomes even more expensive once you start adding a few other bits on such as larger wheels and audio.
Looking at the brochure from this time in 2016 the non PP base price is £3k less on all GTI's than it is at today's starting base cost of a PP, and it was exactly the same in 2017 which saw no price rises. Then in 2018 the multiple price rises and model trimming started, the price jumps and they also trim off the 230 so only a PP can be bought, and in 2019 the manual and 3dr goes.
Go one step further back to 2015 and my brother bought a brand new 5 door manual (no options) and it cost him £23.5k after discount.
Thats not to say other brands have not done the same over the past few years, but the point Moots was making is a valid one... long gone are the days you could walk into a dealer and spend £25k(after discount) on a GTI and walk out happy. It's now much much closer to £30k (after discount) due to price rises and model trimming, and I suspect even more when the MK8 arrives.