There is a detailed Parliamentary Research Briefing Paper dated 5 November 2024 that can be found here
This paper incudes detailed information on flood defence expenditure since 2000.
There is also a National Audit Office report on Resilience to flooding that you can find here
https://researchbriefings.files.parliament.uk/documents/CBP-7514/CBP-7514.pdf.
https://www.nao.org.uk/wp-content/uploads/2023/11/Resilience-to-flooding-.pdf#page=15