Different solutions for different countries. In most european countries the infection rate is already higher than in the UK (often 5 times more). They have to act now, so that their health system are not overburdened in 2-4 weeks. The UK has more time, and now imagine the UK introduces similar measures now.
The probably most feared consequence ist, that everything works, the infection rate goes down, the NHS copes rather "easily" with the pandemic. The population feels safe and that is everything under control.
Mid-April comes good weather around and everybody wants to enjoy themselves and goes out again, visiting festivals, sitting in parks and having a good time.
And BOOM the infections spike again. Cause the overwhelming majority never had the virus, they are getting now infected. And the NHS is again at risk to be overburdened. So the government decides to shut down public life again. How do you think the population will react?
Just say no problem, we will do it again and stay in the summer months home? Or will substantial parts (Youth and young adults, the least at risk age groups) say F*** that I am going to enjoy my life and the summer? So the infection chain starts again.
If its the latter, the government, UK and especially the NHS is in deep s***.
And that is the reason they talk about the right timing of the social distancing measures. Its probably for all out effectiveness a one time only shot. And you want to get it right to protect the most vulnerable and give them the best possible access to medical care without forcing medical personell to make choices who lives and who not.