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?
It depends where you got it. A council parking PCN in London is £160 or £140 for the more serious contraventions and £110 or £90 for the rest; outside London the bands are £70 or £60 for the more serious and £50 or £40 for the rest. Bus lane and moving traffic PCNs in London are £160. ULEZ, the Congestion Charge and the Blackwall and Silvertown tunnels are £180. Nearly all of these halve if you pay within 14 days — and nearly all rise by 50% if you ignore them. Private parking operators are not councils and set their own charge, commonly £60-£100.
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.