Countries used to borrow money to fight wars, paying it back when the war was over. The Thatcher/Reagan era was the start of government borrowing at a time when there was no existential threat. It has continued ever since. Austerity is the right thing to do, but the reason for it is to cut the debt. What's happened, however, is that the Tories have slowed down the amount that the government is borrowing, while cutting vital services and making people suffer. And the debt has doubled. The country is, in reality, bankrupt.
Austerity was a terrible thing to do. It's basically removing money from the economy, which surprisingly enough makes the economy weaker than it would otherwise be. This was a time to borrow to invest, you can pay off the debt once things are booming again.