They print it ! They get banks and pension funds to buy bonds which say, ' Lend us your money and we'll pay it back in X years'. When X years arrive they don't have the money to pay back ('cos they're in debt) so they borrow more, or print it. If the gov needs to borrow £1billion but can only sell £500million in bonds, the gov buys the rest of the bonds. So the gov buys it's own debt with money that it created out of thin air and pays itself the interest. I know - welcome to the cooky land of high finance. Here's the real kicker. The money to pay interest doesn't exist, so they have to create it
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Honestly if you or I tried this kind of thing we would be arrested and chatged with running a Ponzi sheme (look it up)and thrown in jail.
As the created money comes out of thin air - not based on labour, goods, property, precious metals, or wealth creation of any kind - then it is WORTH nothing. In fact, as most money created today is digital, that makes it not even worth the paper it's not printed on.
Do not believe treasury/bank of England figures of 2% inflation, these are highly massaged and 'conveiniantly presented.