Is this a Boris plan to be able to say he's reduced the people living in flats? Kill more of them off so there's fewer people living there?
Do not go out. Not going to a green space won't kill you. Getting covid-19 might.
And in real terms, how on earth are you going to spare anyone to do this? At my place of work some of the people have been working 15+ hours a day to set up emergency relief. That's emergency I need my inulin and I can't get to the pharmacy, or I have absolutely no food left in the house not oh dear I'm a bit low on Wensleydale cheese.
Cards would require someone to produce them and many people to post them, and then someone to check on entry to the space. And how long before people are lending them to friends/relatives and selling on ebay? Probably 2 minutes after the first people get theirs.
And anyway, by the time you'd set it up then we'll all be on lockdown and not allowed out.
If people don't use common sense tomorrow and stop going out except for necessities, that's what is going to happen.