Why is it the case that many people think that the testing criteria of the MOT have no real world backing in law (i.e. something that would result in an MOT failure is not an offence until an MOT flags it up)?
Much (not all) of the MOT is to ensure compliance with key areas of The Road Vehicles (Construction and Use) Regulations 1986 and The Road Vehicles Lighting Regulations 1989. Breaches of these 'roadworthiness' regulations are offences under The Road Traffic Act 1988. The MOT does not ensure full compliance with all sections of these regulations but non-compliance of a testable item is underpinned by law. Driving a vehicle with a testable item that
would fail the MOT is therefore, by definition, an offence.
The Mirrors section of the MOT test is to ensure compliance with S.33 of RV(C&U) Regs 1986...
http://www.legislation.gov.uk/uksi/1986/1078/part/II/chapter/E/crossheading/mirrors/made
A breach of that section is covered by S.42 RTA 1988...
http://www.legislation.gov.uk/ukpga/1988/52/section/42
Penalties for offences under RTA 1988 are provided by The Road Traffic Offenders Act 1988. Schedule 2 of RTOA 1988 lists the maximum penalty for the missing offside mirror (provided the car was first used on or after 1st June 1978) as a fine of Level 3 on the standard scale, i.e. £1,000...
http://www.legislation.gov.uk/ukpga/1982/48/part/III/crossheading/introduction-of-standard-scale-of-fines
The likelihood of prosecution is another matter...