HSE key messages is that trestles can be used if the risk assessment shows that the risk of a person falling and injuring themselves is low and the work on top of the trestle is in short bursts.
Steel or aluminium trestles are used in conjunction with scaffold boards or staging. Guardrail systems are available for trestles and you would need to show that installing a guardrail has been looked at and why it was not considered necessary, if you do not install them.
Possible hire cost: in the region of £20 per week
Set up time 5-10 minutes