Hi,
Usually a rental business begins when letting first commences, which in this scenario would mean that the rental business commenced on 1 April 2023 when the first letting began. It is important to distinguish between activities that are preparatory to letting and those business activities that are part of letting.
As long as the expenses incurred would have been allowed as a deduction if it had been incurred after the rental business started and were wholly and exclusively for the rental business and were of a revenue expense, not of a capital nature, then you are correct, you may claim these as allowable expenses.
As the letting began on 1 April 2023, depending on the payment basis, this would be from the point the business commenced. PIM1900 covers this in further detail as well as PIM1092.
PIM1900 - Deductions: Contents
PIM1092 - Cash basis for landlords: overview
Thanks