This does not show the government in a good light.
Pre-planning for disasters, and testing those plans is responible, but to pick such an unlikely disaster makes TPTB look silly. The fact that the plans would be useful for other large scale disasters tends to be overlooked.
An excercise in responding to a large explosion would be more relevant, for both accidents and terrorist attacks.
We have had a number of large explosions, both industrial accidents and terrorism.

We do have earthqaukes here, but they are very minor, very rare, and the loss of even a single life in an earthqauke in the UK is rare.
http://www.thisislondon.co.uk/standard/article-23443207-earthquake-hits-london.do
Report of the worst earthqauke in England for many years, caused fear and distress no doubt, but a very minor incident compared to events elswhere.