Crown Prosecution Service: Sentencing Manual
It's pretty advanced, so not suitable for everyone!
Many criminal offences are punished by a fine.
For example, damaging property (e.g. smashing things up, breaking windows and painting graffiti) can get you punished by a "level 4" fine:
Level 1 - £200
Level 2 - £500
Level 3 - £1,000
Level 4 - £2,500 (or 3 months imprisonment)
Level 5 - £5,000 (or 6 months imprisonment)
Source: http://www.cans.org.uk/notes/administration-of-justice/treatment-of-offenders/52-maximum-fines-the-a-standard-scalea
Here you can find some easier to understand guides to how the law works in the UK:
Courts, sentencing and tribunals
This is particularly interesting: How sentences are worked out