Without me getting too bogged down in politics, being the thorny subject it already is in its own right, it does superficially seem as though the govt are sounding 'defeatist', but I personally think they (and all the other relevant authorities) just feel an obligation to err on the side of caution by overestimating the possible risks of the virus escalating, regardless of whether they turn out to be right or wrong. If the virus does happen to escalate in the UK in due course, they cannot then be accused of 'covering up' and 'playing down' the potential risks. As the situation is constantly shifting, even the so-called 'experts' are still in the process of learning.
In the meantime, we still need to try to remain calm and heed the advice of the proper 'experts', and not the know-it-all 'fake news-mongers', 'conspiracy theorists' and the like on social media, etc.