that they don't have"
Well yes, but it's worth remembering that actually we're spending it. All this money is spent on the NHS, the roads, defence, education, the environment, and all the other myriad things that make the country what it is, the things that successive governments think we want to keep us happy.
We borrow money because what we pay into the treasury by way of taxation isn't enough to cover our outgoings. There's no great mystery to it, and it isn't a new thing - we've had a national debt since the beginning of the eighteenth century - in 1700 the country was in debt to the tune of £12 million, a huge sum for the time.
Then we borrowed more to finance out activities in the Napoleonic wars, and again in the first world war. Between 1914 and 1918 the national debt increased from around £600 million to about £7.2 billion. We were on our way to huge and permanent national debt, and then came the second world war.
By the time the war was over we were well and truly in the mire - the debt had climbed to over £24 billion, which was at that time 250% of our GDP.
Fighting other nations has contributed as much to our debt as anything else.