Why UK’s Covid-19 strategy is unwise

The UK did not want to cause panic and did not travel. Instead, allowing more UK citizens to get immunity over the infection, herd immunity… But on Mar 15th the UK secretary of state said it is not the strategy, but people over the age of 70 will be told to self-isolate. In my opinion, WHO is already not doing a good enough job to contain the virus, doing less than WHO suggestion which is really unwise. The UK believes over 60% get infected and immunity will eventually happen. When I read this, I could not believe my eyes. How unwise this decision is?