It could be an offence in several circumstances.
Offences under the Fire Safety order usually only arise if a failure to comply (eg wedging a door) actually places one or more persons at risk of death or serious injury in case of fire.
Who would decide whether that was the case? The Court would decide, and in order to bring you to court the Fire Authority would have to read you your rights etc and take the prosecution forward.
However if the Fire Authority have already served an enforcement notice in respect of your wedging doors open then to breach the requirements of the notice is an offence in itself. There is then no need for the prosecution to prove that persons were placed at risk.
Similarly if an alterations notice or a prohibition is in force any breach of this would be an offence.
Where offences are committed under the Fire Safety Order procedings may be taken against any person who has a hand in committing the offence in addition to the Responsible Person.
But please- wedges are a diabolical abuse of a fire door. If it needs to be held open for any reason then please do it in a safe way using an appropriate hold open device that will release on operation of the fire alarm. There are loads of devices on the market to do this, some are safer than others, some doors are more critical than others, you need to identify the correct device for the circumstances of the case. In many situations a device such as a dorgard will be fine, in other cases swing free self closers or magnetic hold open devices controlled by the fire alarm are needed.