I wrote earlier the Gov't is dammed if they do and dammed if they don't, and how that is playing out. Some people seem to think the government is being too restrictive, and others too lax. Some (not just on this forum) are seem to almost accuse the Gov't of spreading the virus.
It is the nature of virus's to mutate, and every time the virus infects a new cell, it combines parts of its (Edit RNA)DNA and it produces a slightly altered versions of the virus. Usually these variations are very minor and it doesn't radically change the virus's modus operandi, but occasionally it does produce a critical change. With our present technology we cannot predict exactly how or when or where a significant change may occur.
When a significantly new strain does emerge, it will only be noticed when in some way it starts to make its self apparnt, such as increasing transmission rate, or causing a rise in a new or more severe symptoms. It requires an approach to wide spread monitoring to gather enough data to start to see changes in trends.
Unfortunately to begin to see new trends it already means some people must have been infected before it can be identified, and to see how severe the changes might be.
The only way to prevent new strains is to prevent the virus from infecting new victims. that requires a lock down to put physical distance between host and potential new victims or producing a immune response that stops the virus from using host cells to reproduce.
I'm in the camp that feels the Gov't has not done enough soon enough in this instance. The present new strain was identified as being able transmit up to 70% more readily by the middle of last week. It must have been obvious that if this strain was widely present in London, then with the prevalence of commuting to and from the Capitol, there is a fantastic opportunity for the new strain to travel to most parts of the UK, and given its transmissibility it is going to be a major problem.
Why they only increased the restrictions in the SE (which was reactive) they should have been proactive and told anyone who had travelled from SE and London to totally self isolate for 10 days.