I'm assuming "preferment" means preference, but I don't understand it. s 6(4) assumes that Defendant completed the offence. Why would've Defendant been charged with just the attempted offence? Why only after charging Defendant with attempted offense, would the Crown rather prefer "an indictment for the completed offence"?
Why wouldn't the Crown just charge all Defendants with both the attempted and completed offences from the outset?
Is it possible to convict of an attempt if the attempt was successful?
Is it a defence to a charge of attempt if the defendant in fact succeeded? The answer (not surprisingly) is that the defendant can still be convicted of an attempted offence. Section 6(4) of the Criminal Law Act 1967 resolves the issue:
where a person is charged on an indictment with attempting to commit an offence or with any assault or other act preliminary to an offence, but not with the completed offence, then (subject to the discretion of the court to discharge the jury with a view to the preferment of an indictment for the completed offence) he may be convicted of the offence charged notwithstanding that he is shown to be guilty of the completed offence.31
31 Webley v Buxton  QB 481 indicates that although s. 6(4) does not apply to summary offences, the common law rule is the same and a defendant can be convicted of an attempt to carry out a summary offence, even if in fact they have committed the full offence.
Herring, Criminal Law: Text, Cases, and Materials (2020 9 ed). p 792.