Hi,
You can find guidance in PIM2500 onwards which states that a rental business begins when letting first commences.
PIM2505 - Beginning and end of a rental business: commencement
Allowable revenue expenditure incurred before the rental business begins can be relieved. You may be able to claim a deduction for these expenses once the letting begins as set out in PIM2505, for an expense to be allowable the expenditure must be incurred wholly and exclusively for the purpose of the rental business and must not be capital expenditure.
As your letting only began in 2023-2024 the expenses would be claimed in that year. As you already complete a Self Assessment tax return you should also report your overseas rental income/expenses on the SA106 Foreign supplementary page.
Thank you.