Re: Never use credit cards.
"why are private companies being allowed to issue currency?"
Guess you don't use stamps, then. Or collect coupons, or use cheques. They're all currency by any reasonable definition of the term - people still use stamps as a reserve currency and for payment. Coupons are self-explanatory.
Cheques are particularly interesting. They're essentially the same as a promissory note and have the same origin as paper money, in credit notes issued by a bank, being a promise to pay the bearer on demand, except issued by a private individual rather than a private corporation.
And of course you must not use any currency valued greater than £1 at any time, as all the paper money in the UK is issued by private companies. The Bank of England is not a government institution and never has been. The banks that issue notes in Scotland and Northern Ireland are also not government institutions and never have been. The government may create fiat money out of thin air these days but all it's actually doing is legislatively requiring the private company of the Bank of England to issue to issue currency that didn't previously exist.
And all contemporary currency, even the shrapnel tinkling away in your pocket, ultimately began as currency issued by a private company.
So. Why are you spending currency issued by a private company if you're so against it?