Her "splashing" on my doesn't compare to the "splashing" I have done on her and her family since
How can you possibly compare the two unless she was earning the same income as you are now? The "splashing" on you could well have been a significantly higher percentage of her disposable income. (For clarity: disposable income = income after basics, not expensive upgraded versions of basics.)
YANBU to wear what you want, but it's not really about that is it?
You could just as well ask "AIBU to eat what I want?" - of course you are, but if you're eating a luscious banquet in front of a starving person it's grotesque.
I initially thought you had spent maybe a couple of hundred pounds on the bag, and thought you could have been more discreet (though friend wasn't very nice). The likely actual cost of the bag is blowing my mind. I suspect it is the same for your friend, if she is struggling financially (even if not tbh, even those I know who are comfortably off would be gobsmacked at spending that much on a bag!)
Something is very very wrong with the world if you can afford to spend more than a lot of people's annual income on a bag. That isn't your fault personally, but your friend who gets the rougher end of the collective bargain is understandably pissed off/frustrated/hurt. She doesn't sound like she's articulating it very well; and it may not be clear even in her own mind, but I bet the simmering effects of inequality are involved.
Even if you have made the "right choices" to end up with your income, it doesn't mean the payoff is remotely proportional. Could we all make the same "choices" as you or would society crumble without lower paid employees? What about those who don't have the same chances, or have a rough start in life that affects their achievement, or are just not very academic or clever? Do they really deserve so many, many thousands less in pay than you?
Your friend may well have been encourgaed to go straight to work after school; indeed this may have a lot to do with a strong work ethic rather than the more relaxed studenty thing (from a certain viewpoint, instilled by friends parents?) Even if she had gone to university, or whatever other "poor choices" you might be referring to - does that guarantee everything will be hunkydory? Not to mention poor choices are generally not apparent at the time or people wouldn't make those choices!
As for "working hard"; you choose to, your friend has to. You have the luxury of choosing a different work/life balance, in return for knocking a figure or two off your income and still being comfortably off. Your friend might be jealous, but my goodness it's justified.
Despite my massive soapbox rant (!), I think you basically sound like a nice person, I just think you're utterly unable to put yourself in your friend's shoes because you live a different world (financially) than most of the UK population! (Just like most of us can't put ourselves in the position of someone starving in a famine.)