When determining whether all or part of a revenue expense is deductible, consideration should be given as to whether the expenditure was actually incurred for the purposes of the rental business. If all the expenditure, or a definite portion of it, was incurred for the purposes of the rental, then this portion is allowable. The purpose of the expenditure is key.
If at the time the expenditure was incurred there was no property business and no property business purpose then the expenditure would not be allowable, there is more guidance at:
PIM2010 - Deductions: general rules: applying the wholly & exclusively rule
PIM2505 - Beginning and end of a rental business: commencement