//If they weren’t convicted, how can the police claim the two men trafficked women and made them work ad sex slaves?//
They can mention their suspicions, among which may be the suspicion that they have committed offences. Hence the request for the order.
It is not unusual for the police to seek prevention orders against individuals whom they believe either have or are likely to commit offences but against whom they cannot secure criminal convictions. The burden of proof to convince a court of their suspicions (thus allowing the order to be issued) is far lower than that needed to secure a criminal conviction (being to the civil standard “on the balance of probabilities”). You may also notice that the order does not specifically prohibit the commission of offences but prohibits a wider range of activities. These two, for example, must not "...arrange travel or transport into or within the UK for anyone other than themselves, including for work." So the police will not have to prove involvement in trafficking. Merely arranging travel (for anybody for any purpose) - which of course is not otherwise illegal - is effectively criminalised for this couple.
Domestic Violence Protection Orders and Restraining Orders are similar in nature to those mentioned here. Recipients of DVPOs are not prohibited by them from beating up their partner. There is no need for an order for that as it is illegal. They are usually prohibited from (among other things) contacting or visiting the alleged victim and merely doing that will see them face criminal charges.