Causes of recent UK Deficit
The chart you are looking at is off by a year: it should have shown about £10 billion in 2007 and almost £100 billion in 2009.  See the ONS figures for the "Public sector current budget deficit, excluding public sector banks" and click on "table" and "year"

It is more common to look at "Public sector net borrowing, excluding public sector banks" but the story is similar: about £37 billion in 2007 and about £153 billion in 2009

There was a reduction of about £25 billion in central government current receipts  and an increase of about £60 billion in central government current expenditure  plus some other things happening.  Falling receipts included lower corporation and income taxes and part of a 12 month cut in VAT, while increasing expenditure included support for banks and increased numbers of people claiming unemployment and other benefits.  Interest payments (part of expenditure) actually fell by about £4 billion due to lower interest rates, though they rose the following year due to the increased debt

You can find more data in the ONS Public Sector Finances time series dataset spreadsheet