First, you'll need to find a local shop, petrol station or what have you with a Paypoint register. These are the ones through which you can pay bills, renew your TV licence and add funds to your pay-as-you-go mobile or prepaid card then and there. Head to Amazon's top-up site on your phone, or track down the equivalent page on the retailer's mobile app, and grab yourself a unique barcode.
Get the shopkeeper to scan that barcode, hand over between £5 and £250, and it'll immediately show up in your Amazon gift card balance. It's tantamount to using the money to buy a gift card, but with less steps and thus, less hassle -- but beware any deposits are non-refundable. You may remember Amazon launched a similar service in the US earlier this year, albeit with the catchier title of "Amazon Cash."
It may seem strange to go through this whole process when it's infinitely more convenient to make card payments, but not everyone has a bank account and some people would still just rather not hand over financial details to a website. Amazon is in the business of selling stuff, so if there's a demographic that can or will only use cash, you bet Amazon is gonna make sure they can spend it online.