The UK Goverment borrows money on the open market, the agency responsible for raising money for HM Government is The UK Debt Management Office.
The DMO can issue Gilts or Treasury Bills to raise cash. These are effectively “I Owe You’s” that mature in the future.
Looking at May 2013, we had 4 Gilt auctions:
Wednesday 8-May-2013: 0 1/8% Index-linked Treasury Gilt 2044: Raised: £1,335.4556 Million
Tuesday 14-May-2013: 1¼% Treasury Gilt 2018: Raised: £5,297.7305 Million
Thursday 16-May-2013: 3¼% Treasury Gilt 2044: Raised: £2,727.0145 Million
Wednesday 29-May-2013: 4¾% Treasury Stock 2015: Raised: £1,922.4556 Million
This equates to £11.282 Billion for May 2013 on issuing gilts only
Or to put it simply, The UK Goverment borrowed (using Gilts) £363 million a day to bridge the gap of its income (tax collection) and government spending.