What are allowable expenses
Expenses you incur "wholly and exclusively for the purposes of your trade". Typical expenses include tools, materials, safety equipment, payments to subcontractors and overnight temporary accommodation.
What about travelling costs
Commuting to an "ordinary workplace" is not considered a relevant expense. The rules for travel to a "temporary workplace", however are different. The definitions for a temporary workplace are not exactly clear. This is be a subjective area and as always, it will depend on your personal circumstances.