//See, not one mention of contravening regulations being described as a criminal act.//
I cannot recall any statute mentioning a “crime” or a “criminal act”. Any that I’ve looked at contain “…is guilty of an offence” (or similar). Even the section of the RTA describing the most serious of motoring offences (Causing death by dangerous driving) makes no mention of a “criminal act”:
“A person who causes the death of another person by driving a mechanically propelled vehicle dangerously on a road or other public place is guilty of an offence.”
//Moreover, in compliance with the law, from now on I shall take off my seatbelt when performing a reversing manoeuvre.//
You don’t have to remove your seat belt when reversing in order to comply with the law. You can keep it on and still remain legal. The legislation simply provides an exception to the law for those who wish to remove their belt.
//Go to court and contest the alleged infringement. Fair enough.
To abolish FPNs for minor offences would be retrogressive.//
Who is talking of abolishing them? However, you should bear in mind that the offer of a Fixed Penalty (rather than proceeding immediately to criminal prosecution) is entirely at the discretion of the police. A recipient has no right to a FP and has no right of appeal if he is prosecuted rather than being offered one. He can, however, if he is prosecuted, try to persuade the court that a fixed penalty would have been appropriate in all the circumstances and request to be fined at a level equivalent to the fixed penalty.