The UK National Debt is the total amount of money the British government owes. The UK government borrows from creditors who buy gilts (UK Sovereign Bonds). Thus the ability for the UK to bridge the gap from its public expenditure and its tax revenue comes from the fact it is able to borrow on the fixed income (bond) market.

In 2014, the public sector net debt was £1,268.7 billion, this is just over 75% of gross domestic product (GDP). [UK GDP is about £1673 Billion (approx.)]

But let’s keep things in context:

Japan for example has a National debt of 225%,
Italy is over 100%.
US is about 100%

Also after Second World War, the UK debt  was over 180% of GDP.

However with all borrowings, the interest is an issue, one has to pay interest on the debt.
Thus the cost of National debt is the interest the government has to pay on the gilts it auctions.

In the next 5 years this is the estimate on what could have to be paid:

2014-15: £52 Billion
2015-16: £57 Billion
2016-17: £62 Billion
2017-18: £68 Billion

These are huge numbers, on money that brings no economic benefits such as creating jobs. It is just payments to creditors.

The only way to reduce the debt is via economic expansion and government spending cuts.

Final point on government debt, perhaps the number is higher, if you consider PFI programmes, or perhaps pensions that the government is obliged to pay and the liabilities of bank loans from the old Northern Rock (now UKAR) it has to guarantee. So perhaps the UK National Debt is actually well over 100% of GDP