The security guard, acting for the (now former) owner of the property doesn't know the new ownership of the property and asks for evidence Bob now owns the property. The security guard is free to ask.
Bob cannot produce a receipt for his purchase of the property. The security guard asks which checkout Bob used, so that the security guard can check its records for the transaction. Bob doesn't know specifically and says the checkout was one of three. The security guard asks Bob to remain while each of the three checkouts is checked until Bob's transaction is discovered (or not). The security guard is free to ask.
Bob is legally free to leave with his property but the security guard may think he has reasonable grounds for suspicion of shoplifting and decide to try to detain Bob on suspicion of shoplifting until the ownership of the property is established.
As any member of the public, the security guard may use "as much force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large." (s3 Criminal Law Act 1967)
Bob is also free to leave without his property. In this case, as there could be no grounds for suspicion of shoplifting, only attempted shoplifting, it seems unlikely that any force would be reasonable.
Of course, depending on retailer policy the security guard may be allowed to ban Bob from the premises if Bob doesn't cooperate.