Why has this become a thing? Spend twenty-plus quid so everybody can see you're 'doing it for charity', when what you are actually doing is spunking cash on something you will wear for between 1 and 5 days in a year before dumping it in the back of the wardrobe or binning it to buy another one next year?
People who can't afford that twenty pound feel under pressure/disapproval for 'not joining in, it's FOR CHARITY'. Somebody makes a hell of a lot of money for the jumpers and chucks a couple of quid at most to a charity. You've just given them TWO QUID, that's all. The seller still has eighteen quid of your money and you still look like a twat.
If somebody wants to do something for charity, wear your usual jumper, take that twenty quid you would have wasted on some synthetic monstrosity with bells or reindeer ears and buy some food to shove in the collection bins for the local foodbank, don't take fucking photos with all your mates wearing disposable jumpers, thinking you're Really Making a Difference.