PCNs are civil, not criminal
A Penalty Charge Notice is a civil debt, not a criminal fine. Unpaid PCNs do not create a criminal record. They are issued under the Traffic Management Act 2004 by local councils, by Transport for London for red routes / bus lanes / Congestion Charge / ULEZ, and by some private parking operators (though private operators technically issue "Parking Charge Notices" — same initials, different legal basis).
How much is a PCN?
Council PCNs are usually £60 or £80 with a 50% discount for paying within 14 days. TfL PCNs and ULEZ / Congestion Charge penalties can be £160-£180. Private parking operators commonly charge £60-£100 with a similar early-payment discount.
What happens if I ignore a PCN?
Council PCN: the council issues a Notice to Owner, then a Charge Certificate (which raises the amount by 50%), then registers the debt with the Traffic Enforcement Centre and can pursue it via court bailiffs. Private PCN: the operator can sue you in the County Court — and increasingly does. Ignoring is almost always a mistake.