While we have heard a great deal in recent weeks about the unprecedented nature of the floods, this article by Terry Marsh of the National Hydrological Monitoring Service should calm things down a bit:
Tidal flooding is expected to increase as thermal expansion of the oceans, supplemented by meltwater from glaciers and ice sheets, increases sea levels. The risk of flooding from groundwater may also increase if winter rainfall rises appreciably.
However, the outlook in relation to fluvial (river) flooding is much less clear cut. This can be well illustrated by flow patterns in the Thames, for where the recent flooding has been most extensive. The Thames we have one of the longest continuous flow records in the world. Systematic flow measurement began at Teddington in 1883; there is also a wealth of documentary evidence relating to historical floods.
You need to read the whole thing.