Interesting. On that basis do they have any right to prevent you walking out if they suspect you are shoplifting?
None. There have been court cases where a supermarket has detained someone unlawfully, and retailers have lost.
A friend works for one UK supermarket in a security capacity, and he routinely carries out anonymous inspections to see how the security react to situations such as this. They will send someone to do certain things, such as walking through the detectors with a security label with the intention of setting off the alarm. The point is to see how security reacts, and if physical force is used, the security guard will lose their job.
Their policy is clear: they may only detain someone with force *if* the suspect has been observed taking something without payment, and that the person has been clearly observed with the item from the moment of taking it until they leave the store. If they don't have uninterrupted visual confirmation that the person took and is still in possession of the item, then they are strictly prohibited from physically detaining anyone.
In practice, they don't observe the CCTV constantly, so the security guards rely on their power of persuasion 'please come with me, Sir' rather than on actual evidence. Most shoplifters will quickly confess as a result of being questioned, but if someone simply keeps on walking, there's little that the store can do. Store security cannot stop/search you without your consent, and if they attempt a citizen's arrest, then they absolutely must have reasonable grounds for doing so. The mere suspicion of having stolen something is not enough: they need to have something more than 'that bloke set off the alarm'.
For me, I simply keep walking. I don't engage with them, I don't waste my time discussing the situation, I simply keep walking. I've had one security bloke threaten to arrest me, although he quickly backed down when I calmly asked him if his employer would cover his legal expenses.