If you borrow the very first £1 in existence and you promise to pay it back. Plus another £1 of interest. Where do you get the second £1 from?
You make something with the first £ that's worth £2. Problem is we don't make anything worth £2, so we get our money by advising two people, one of whom lends the other a £ so something can be made worth £2.
I think that's how the world economy works anyway.
Where the first pound comes from is anyone's guess. We probably got ours by winning the game of 'musical (army and navy) chairs' in the second half of the last millennium and nicking all the gold that the Spanish nicked of the Incas. Then gormless broon sold all the gold for a tin of magic beans, the Chinese giant smelled the blood of an Englishman and came down the beanstalk, nicked everbody's gold and forced us to make a load of cheap sh*tty plastic stuff that we then buy with money they lend us.
That may have been some weird dream I had, but then life is but a dream.