I am not sure about the situation in GB, but over here in Germany a main reason for the floods worsening is surface sealing. The more ground is sealed up by buildings, roads, parking lots etc, the less water can filter into the soil. Consequently, it has to run off the surface, such flooding the sewers and rivers.
The overall amount of water stays the same, but it doesn't get sucked up by the soil anymore and then given off slowly, but runs into the rivers almost immediately, causing very quick and high floods which fortunately also ebb off very quickly.
A second factor in the problem is that many rivers and streams got straightened to make more room for farms and settlements. This results in a shortening of the riverbeds. They still have to deal with the same amount of water, though, and so the water level rises.
A side effect that worsens the floods even more is that a curved riverbed slows down the water speed while a straight one is like a highway for the water. This increases the speed of floods, making it difficult to escape them in time.
What we need is more swamps and wetlands that act as sponges, a complete stop of channelizing rivers and streams and nationwide restaurations of the original curved river and stream structure. Plus a lot of retention basins and ponds.