In theory, a person can be imprisoned for not paying priority debts, which include:
•council tax arrears
•court fines
•maintenance arrears
•income tax or VAT arrears
A debtor can only be sent to prison for non-payment of priority debts if they have deliberately refused to pay their debts rather than having a genuine inability to pay them.
For none priority debts you can't be sent to prison for unless fraud is involve AFAIAA.
When someone is fined the amount should be based on income. At the moment for example, the fine for parking is the same if you are on or below the minimum wage or a multi millionaire.
Same for fixed penalties.