Preliminaries are the costs of running the project as a whole rather than any measured work item: site management and supervision, accommodation and welfare, cranage and hoisting, temporary services, security and the like.
They are priced over the planned duration, which is why time is money in a literal sense: when a project runs longer, prelims keep being incurred, and extended or thickened prelims are usually the largest head of a prolongation claim.
Proving them requires records of the resources actually maintained on site through the delay period, which is another argument quietly won or lost by daily records.
Construction Metric keeps that daily resource record automatically for the life of the job, on plans from £199 a month.