Hi,
The exchange date takes precedence over the completion date. It is up to you to decide how you wish the landlord insurance to be claimed as an expense. You could claim it for 2023-24, which would create a loss for the property to 5 April 2024 as you have no rental income to set the expenditure against. Alternatively, you could wait and include the expenditure in 2024 to 2025, and set the expenses against the rental income you received in relation to that property.
PIM2505 - Beginning and end of a rental business: commencement.
As the property was purchased on 5 April 2024, you should show 5 properties when completing your Self Assessment tax return.
Thank you.