Pre-rental expenses of a revenue nature can be deducted as an expense against profits made once the property is rented, and income has been received. Details of this can be found here:
PIM2505 - Beginning and end of a rental business
There are two types of expense usually incurred. Any expenses of a revenue nature can be claimed against rental income and any expenses of a capital nature can be claimed against any future Capital Gain. Guidance and examples on which type the expense comes under can be found here:
PIM2030 - Deductions: repairs: is it capital?
Additional guidance on capital expenditure can be found here:
Capital Gains Manual