So, stepping back from this specific situation because Fyke did not have objectively reasonable grounds to believe, and speaking about petty theft more generally:
Parliament has not set a hard floor of value below which theft is not criminal. Theft is theft, however nominal theft value or how wealthy the victim. Theft is always going to be arrestable. In fact, the public has demanded that shoplifting and retail theft be taken more seriously, and that store staff and security be more empowered and protected in combating it. The power for a non-peace officer to arrest a shoplifter has been reaffirmed. Generally, the public tends to show a lot of frustration when police don’t take petty theft seriously. But then when we do arrest and charge, others think we’re too heavy handed.
How many members get sent to a call will always depend on how it triaged against what else is sitting in the patrol queue, and the degree of risk. If members are tied up at something more serious, lucky day for the shoplifter. But if there are units free and someone has kept continuity on a suspect, why not attend, investigate to the degree necessary, and, if grounds exist, arrest? Bear in mind an arrest is just that; it doesn’t mean a charge. But it does serve to identify the suspect(s), and a lot of times, minor criminality, promptly acted upon, reveals more significant crimes, or breaches of bail, probation, etc.
But in the eye of the public, it’s damned if we do, damned if we don’t, because the public is very disparately factionalized with regards to what kind of policing they want to see. It is what it is.