Evaluation of government spending
There is a danger of moral hazard. If the government promise to secure all land from flooding, it may encourage people to build and move onto flood plains. If people wish to live on flood plains / near rivers, arguably they should be prepared to pay part of the cost.
Many of those affected by the floods are relatively more prosperous than the rest of the country. There is an argument about limiting the transfer of funds from the average tax payer, to wealth homeowners living by the River Thames.
But, it should be remembered flooding doesn’t just affect rich people with mansions on the Thames – it is a rather lazy generalisation. I know every income bracket in Oxford is being hit by flooding.