How much we owe depends on how you do the figures. UK national debt is about £900 million.
You can get over 1 trillion if you add in a bunch of things like pensions that we don't have to borrow for now (that's what the Centre for Policy studies have done - they're a right wing think tank)
Think of it this way though - £1 Trillion is about £16,000 per person.
Now that's quite a lot of money but it's nothing compared to what many people owe already - and it's nothing compared to what we owed after the war.
It also doesn't account for assets that we spent the money on - The Government stake in Llyods -TSB alone is about £15 Billion
What is important is to make sure that figure is going down and not up and a lot of doing that is making sure that the economy grows.
Growing economy -> more tax revenue + less benefit claimants
Cutting spending too harshly will give you a vicious circle of more unemployment and less tax revenue that's what happened in the early 80s
But back to your original question.
Governments issue bonds called gilts - these are promisses to pay back with interest in x years.
These are bought and traded by investments houses and banks and pension firms.
If you want a real shock realise this
We borrowed all this money to bail out the financial sector and prevent a depression.
This same financial sector is now saying "you've borrowed too much money - if you don't reduce it, we'll downgrade your credit rating"