A few years before the Apocalypse, the city of London had a serious problem. The direct threat of nuclear attack prayed on the minds of 8 million people, all looking for a way to survive. The serious preppers knew to get out of the city but those without a plan relied on the authorities of our capital to help them survive. The London Underground network was chosen to house only a potential 100,000 people, but wartime propaganda put the total at a much higher number. Those that did make it down into the Tube on that fateful day the bombs fell found a hard way of life, surviving on rations and little room to live. Those stations too close to the surface just didn't protect against radiation exposure and those further underground slowly leaked down. The low level radiation exposure over time wrought slow, agonizing misery on those below and soon peoples' hair and skin began to fall apart. These people quickly became mutants of a very particular kind: radiation poisoning sufferers. Rather than death, these victims had their life extended but with pain, irritation and infection to deal with.
Once rations began to run out, gangs formed of the particular Underground stations they lived in began to fight with each other, for resources like food and electricity. After several horrible years, one man stepped up to quell the butchery of the London Underground. Freddie Noone, pre-war businessman, promised access to his personal stores of food if people could "pay" him with Meantime Brewery bottlecaps and thus he created the bottlecap 'standard'. 5 caps could buy you an unlabelled tin of food while 10 could buy you a labelled tin. A small economy, tied to the value of food, was born.
Once the ghouls tunnelled out beyond London and the '25 Orbital Motorway, headed by Mr Noone, the influence of the bottlecap spread to East Anglia and the midlands, giving scavengers around the country a workable system of bartering and money. You'll find Ghouls in amongst most settlements now, but ask most of them and they'll tell you they came from the Underground of London.