The "Budget" is 702 billion.
The expense is 698 billion.
Now the department expenditures is 702 billion.
So they spend 4 more billions then they got.
At that point I laugh.
No, look closer. The deficit is £55 billion. What I think you're confused about is that there's a £698bn allocation that seemingly magically feeds into a £702bn budget for department expenditures. However the £4bn difference comes from HM Treasury (it's quite a thin line at the bottom)