It is only a "one off thing" in hindsight. I recall well the UK deficit was 14% of GDP each year. That is not sustainable. The point you are missing is that if the government had said "YOLO, lets just increase the deficit to 20% and invest in infrastructure!" then the markets would certainly not have continued to lend money at 1%. That's the path towards an Argentina style situation.
Once you've bailed out the banks, then what? You're going to bail them out again? (No, because they're now publicly owned). There was no reasonable scenario where that was not a one-off increase in the debt.
We were borrowing at 1% (basically free money because inflation was higher) and we might have spent that on growth policies, but George Osbourne thought this was a good way to reduce the size of the state, a political decision.
