The theory is that once the lockdown is finished, the virus spreading will go exponential again. In which case, you need another lockdown. And so on.
And no country has the economical capabilities to stay in lockdown until end of 2021, when we can assume mass vaccination will be possible (maybe a bit earlier like summer 2021).
I am not saying that I think this is the case, but this seems to be the logic behind it. And well, I am a bit afraid that they are right, otherwise UK's plan (and US, Netherlands, Sweden) is nuts. I am afraid that they have checked all the possibilities and have come to that conclusion.