Wow they are in bad nick for 4 years old
even the bolts and centre caps look shot!
Agree - they do look quite bad..
A painted / powder coated finish like the OP has on their wheels is usually very durable and can withstand most of what the British weather throws at it.. The powder coated alloys on my current VW have been on the car for almost four years and three months - so a similar age to the OP’s wheels - and they’re virtually indistinguishable from new. It’s usually diamond cut alloys that have poor durability and can suffer from premature corrosion issues.
i had corrosion issues on a couple of the diamond cut alloys around the centre caps on my last car (Mk6 Golf). The centre caps on those wheels were replaced under warranty, and I‘m reasonably certain that the dealer removed the centre caps by levering them out with something like a small bladed implement from the front of the wheel, and they damaged the lacquered surface around the centre bore in the process. Those wheels then suffered from white worm corrosion around the centre bore.
Is it possible that the centre caps on your wheels have been removed using a similar method to that used by the VW dealer on mine, and the paint / powder coating has been damaged in the process? If so and the damage was right on the edge of the wheel’s centre bore, you’d be unlikely to notice the damage until it developed into bubbling and blistering as yours has. Also, harsh cleaning products could damage the finish on alloy wheels over time. Whether or not harsh / inappropriate cleaning products would degrade the finish so that it suffers from bubbling and blistering I don’t know, but could possibly be a contributory factor if something like an acid based cleaning product has been used.
Alternatively, would you know if your wheels have ever been refurbished? If so and if the quality of the refurb was below par, then it’s possible they could prematurely suffer from a deterioration in the finish.