Who said it?
The government said they wanted to keep deaths below 20,000 so its no surprise that there will be some talk around this number. Including talk that involves hope and optimism.
Personally I would not have set a target like that, although I recognise that it is helpful to give some sort of indication to the public as to what sort of scale of things you are anticipating.
I'm not a big fan of premature hope nuggets myself, although I can see why they are considered important, especially during a lockdown when you are trying to encourage people that their actions are making a difference.
I dont have all the numbers that governments have, so it is also possible that I will not spot more genuine and sustainable hope nuggets as soon as they arrive on the scene, I might mistake them for false hope or stuff that isnt based on actual data.
As for the actual number of UK deaths to expect, I dont have a number in mind myself. Especially when Italy and Spain, who are ahead of us, are not really slowing their number of deaths in a way I could draw conclusions from yet.