Hello
@Millercool21 I'll share my own insight. I was a store manager with two UK retailers, end-1990 to start of 1998; in Oxfordshire and then at various central London locations.
Both my retail orgs used third party security companies for in-store uniformed and plain clothes security personnel.
All plain clothes and uniformed personnel had ID on them. They always showed their IO to any individual they spoke to.
No security personnel apprehended or accused a person of shoplifting, without:
a) the security personnel themselves witnessing,.or
b) having been alerted by a store staff member who themselves had witnessed
an act of shoplifting or malicious damage to store property.
if a person had been witnessed thus, then the security personnel - usually with one or more store staff - would stop the individual only after they'd left the premises. Then the security personnel would explain what had been witnessed and they'd ask the individual for their version of events. No use of force was permitted, in stopping a person.
Security personnel did not search people, if needed, the police were contacted to attend and to perform a physical search.
If there was any doubt, then no accusation was made to that individual. The security personnel might, however, keep close eyes on the individual during future visits.
Store staff were always instructed to make no attempt at apprehension or accusation. Even more so, after the sad fatal stabbing of John Penfold at Woolworths Teddington, in the mid 1990s, when he tried to stop one shoplifter; and a further (non-fatal) stabbing of one of my London managerial colleagues the following year.
So, in relation to your experience, your best course of action would be to make an M&S staff member aware of what the individual said to you. That would help M&S identify if that person was a) bogus or b) plain clothes security personnel with a lack of competency